Ethylene oxide process

ABSTRACT

This invention relates to a process for the production of ethylene oxide by contacting ethylene with an oxygen-containing gas in the presence of a catalyst comprising silver, a promoter of an alkali metal and a promoter of rhenium supported on a porous refractory support.

This is a division of application Ser. No. 926,026 filed Oct. 31, 1986now U.S. Pat. No. 4,761,394.

FIELD OF THE INVENTION

This invention relates to supported silver-based catalysts for theproduction of ethylene oxide.

BACKGROUND OF THE INVENTION

Supported silver catalysts have long been used in the conversion ofethylene and oxygen to ethylene oxide. The use of small amounts of thealkali metals, K, Rb and Cs, were noted as useful promoters in supportedsilver catalysts in U.S. Pat. Nos. 3,962,136, issued June 8, 1976 and4,010,115, issued Mar. 1, 1977.

U.S. Pat. No. 3,844,981 issued Oct. 29, 1974, U.S. Pat. No. 3,962,285issued June 8, 1976 and British Pat. No. 1,325,715, published Aug. 8,1973, disclose the use of silver-rhenium ethylene oxide catalysts. Inthese patents a high surface area silver derivative such as silver oxideis impregnated with a rhenium solution and subsequently reduced toprovide metallic rhenium alloyed with the silver. U.S. Pat. No.3,762,285, patent discloses the use of KOH to precipitate Ag₂ 0 fromAgNO₃. There is no disclosure in the patents of the use of suitableinert supports such as porous refractory supports. U.S. Pat. No.4,584,921, issued Oct. 22, 1985, discloses the use of rhenium insilver-supported ethylene oxide catalysts. In this reference, therhenium is first placed on the support in the form of finely dividedmetal particles and the silver is subsequently deposited on the outersurface of the particles. U.S. Pat. No. 3,316,279, issued Apr. 25, 1967,discloses the use of rhenium compounds, particularly ammonium and alkalimetal perrhenate for the oxidation of olefins to olefin oxides. In thisreference, however, the rhenium compounds are used unsupported alongwith a reaction modifier (cyanides, pyridines or quinolines) in a liquidphase reaction. U.S. Pat. No. 3,972,829, issued Aug. 3, 1976, disclosesa method for distributing catalytically active metallic components onsupports using an impregnating solution of catalyst precursor compoundand an organic thioacid or a mercaptocarboxylic acid. Catalyticallyactive metals include metals of Groups IVA, 1B, VIB, VIIB and VIII,including rhenium and which may be in either the oxidized or reducedstate. However, promoting amounts of rhenium in combination with silverand promoter amounts of alkali metal on a porous refractory support arenot suggested. U.S. Pat. No. 4,459,372, issued July 10, 1984, disclosesthe use of rhenium metal in combination with a surface metallated (usingTi, Zr, Hf, V, Sb, Pb, Ta, Nb, Ge and/or Si) alumina or silica. U.S.Pat. No. 4,005,049, issued Jan. 25, 1977, teaches the preparation of asilver/transition metal catalyst useful in oxidation reactions. In thisinstance, the silver serves as both a catalyst and a support for thetransition metal co-catalyst. In U.S. Pat. No. 4,536,482, issued Aug.20, 1985, catalytically active metals such as Ag and Re are cosputteredalong with a cosputtered support material on a particular support. Noneof these references disclose the use of a promoting amount of rheniumwhich is present on a silver-based, alkali-doped supported catalyst.

SUMMARY OF THE INVENTION

This invention relates to a catalyst for the production of ethyleneoxide from ethylene and molecular oxygen in the vapor phase whichcatalyst comprises a catalytically effective amount of silver, apromoting amount of alkali metal, and a promoting amount of rheniumsupported on a porous refractory support. In a preferred embodiment, thealkali metal is a higher alkali metal of potassium, rubidium, cesium ormixtures thereof and is present in an amount ranging from about 20 toabout 1500 ppm by weight of the total catalyst and the rhenium ispresent in an amount ranging from about 0.2 to about 5 millimoles ofrhenium per kilogram of total catalyst. The rhenium may conveniently bea form of rhenium which is extractable in a dilute aqueous alkali metalhydroxide solution, particularly a 20 millimolar sodium hydroxidesolution. In a preferred embodiment the instant combination of silver,alkali metal promoter, rhenium promoter and support affords higherselectivities, particularly higher initial selectivities to ethyleneoxide at a given oxygen conversion level than is obtained under the samereaction conditions with the same combination of silver and support andnone or one of the promoters selected from rhenium and alkali metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows optimized initial selectivity versus cesium promoterconcentration for a catalyst of the instant invention containing rheniumand for a catalyst not containing rhenium thereby illustrating theenhanced initial selectivity obtained with the instant catalyst. FIGS.2-7 show pore size distribution curves for carriers A-F of Table I.

DETAILED DESCRIPTION OF THE INVENTION

Generally, in the vapor phase reaction of ethylene with oxygen toproduce ethylene oxide, the ethylene is present in at least a doubleamount (on a mol basis) compared with oxygen, but frequently is oftenmuch higher. Therefore the conversion is calculated according to the molpercentage of oxygen which has been used in the reaction. The oxygenconversion is dependent on the reaction temperature which latter is ameasure of the activity of the catalyst employed. The value T₄₀indicates the temperature at 40 mol percent conversion of the oxygen inthe reactor and the value T is expressed in ° C. This temperature ishigher when the conversion of oxygen is higher. Moreover thistemperature is strongly dependent on the employed catalyst and thereaction conditions. The selectivity (to ethylene oxide) indicates themolar amount of ethylene oxide in the reaction product compared with thetotal molar amount of ethylene converted. Herein the selectivity isindicated as S₄₀, which means the selectivity at 40 mol percent oxygenconversion. The selectivity of silver-based ethylene oxide catalysts candecrease over a period of time of usage. When comparing the selectivityperformance of various silver-based ethylene oxide catalysts, it isimportant that the selectivity value be measured at approximately thesame period of time of usage under the same or similar reactionconditions. As used herein, "initial selectivity" will refer to theselectivity of ethylene oxide catalysts when measured at a givenconstant oxygen conversion level of 40% at a gas hourly space velocityof approximately 3300 and when measured after the catalyst has beenplaced on stream for about 16±4 hours. Unless otherwise noted, all thatare provided in the examples provided herein are initial selectivities.

In broad general terms the catalysts of the instant invention areprepared by impregnating porous refractory supports with silver ions, orcompound(s), complex(es) and/or salt(s) dissolved in a suitable solventsufficient to cause deposition on the support of from about 1 to about25 percent by weight, basis total catalyst, of silver; the thusimpregnated carrier is then separated from the solution and thedeposited silver compound is reduced to metallic silver. Also depositedon the support either prior to, coincidentally with, or subsequent tothe deposition of the silver will be suitable ions, or compound(s)and/or salt(s) of alkali metal dissolved in a suitable solvent. Alsodeposited on the carrier either prior to, coincidentally with, orsubsequent to the deposition of the silver and/or alkali metal besuitable rhenium ions, or compound(s), complex(es) and/or salt(s)dissolved in a appropriate solvent. Detailed preparative techniques arediscussed herein.

The support or carrier employed in these catalysts in its broadestaspects is selected from the larger number of conventional, porousrefractory catalyst carriers or support materials which are consideredrelatively inert in the presence of the ethylene oxidation feeds,products and reaction conditions. Such conventional materials are knownto persons skilled in the art and may be of natural or synthetic originand preferably are of a macroporous structure, that is, a structurehaving a B.E.T. surface area below about 10 m² /g and preferably belowabout 3 m² /g. Very suitable supports comprise those of aluminouscomposition. Examples of supports that have been used as supports fordifferent catalysts and which could, it is believed, be used as supportsfor ethylene oxide catalysts are the aluminum oxides (including thematerials sold under the trade name "Alundum"), charcoal, pumice,magnesia, zirconia, keiselguhr, fullers' earth, silicon carbide, porousagglomerates comprising silica and/or silicon carbide, silica, magnesia,selected clays, artificial and natural zeolites and ceramics. Refractorysupports particularly useful in the preparation of catalysts inaccordance with this invention comprise the aluminous materials, inparticular those comprising alpha alumina. In the case of alphaalumina-containing supports, preference is given to those having aspecific surface area as measured by the B.E.T. method of from about0.03 m² /g to about 10 m² /g, preferably from about 0.05 to about 5,more preferably from about 0.1 to about 3 m² /g, and a water pore volumeas measured by conventional water absorption techniques of from about0.1 to about 0.75 cc/g. The B.E.T. method for determining specificsurface area is described in detail in Brunauer, S., Emmett, P. H. andTeller, E., J. Am. Chem. Soc.,60, 309-16 (1938).

Certain types of alpha alumina-containing supports are particularlypreferred. These alpha alumina supports have relatively uniform porediameters and are more fully characterized by having (1) B.E.T. specificsurface areas of from about 0.1 m² /g to about 3.0 m² /g, preferablyabout 0.1 m² /g to about 2.0 m² /g and (2) water pore volumes of fromabout 0.10 cc/g to about 0.75 cc/g, preferably from about 0.25 cc/g toabout 0.55 cc/g. Typical properties of some supports found useful in theinstant invention are shown in Table 1. Suitable manufacturers ofcarriers comparable to those in Table 1 include Norton Company andUnited Catalysts, Inc.(UCI).

                                      TABLE 1                                     __________________________________________________________________________    Carrier         A   B   C   D   E   F                                         __________________________________________________________________________    B.E.T. Surface Area, m.sup.2 /g.sup.(a)                                                       0.21                                                                              0.42                                                                              0.42                                                                              0.48                                                                              0.57                                                                              2.06                                      Water Pore Volume, cc/g                                                                       0.26                                                                              0.36                                                                              0.41                                                                              0.49                                                                              0.44                                                                              0.65                                      Crush Strength, FPCS, lbs.sup.(b)                                                             100%                                                                              97% Avg 21                                                                            90% 90% No                                                        20 lbs                                                                            15  Range                                                                             14  15  Data                                                              15-30                                                 Total Pore Volume,                                                                            0.26                                                                              0.42                                                                              0.42                                                                              0.46                                                                              0.42                                                                              0.65                                      Hg, cc/g.sup.(c)                                                              Average Pore Diameter,                                                                        620 560 640 550 770 1000                                      Hg, Å.sup.(c)                                                             Median Pore Diameter,                                                                         3.7 2.7 3.4 3.4 2.4 2.5                                       Hg, microns.sup.(c,e)                                                         Percent Pore Volume in Pores                                                                  90.0%                                                                             88.5%                                                                             89.5%                                                                             89.1%                                                                             91.5%                                                                             94.1%                                     Greater Than 350 Å.sup.(c)                                                Percent Pore Volume in Pores                                                                  87.0%                                                                             82.5%                                                                             83.4%                                                                             82.3%                                                                             83.5%                                                                             61.0%                                     Greater Than 1 Micron.sup.(c)                                                 % Wt. Alpha Alumina                                                                           99.5                                                                              98  98.5                                                                              98.5                                                                              98  70-75                                     Water Leachable Na, ppmw                                                                      12  53  21  24  18  No                                                                            Data                                      Acid-Leachable Na, ppmw                                                                       40  96  87  51  45  No                                                                            Data                                      Water-Leachable K, ppmw                                                                       5   22  21  22  10  No                                                                            Data                                      Acid-Leachable Fe, ppmw                                                                       2   5   No  1   5   No                                                                Data        Data                                      % Wt. SiO.sub.2 .5  2   1.5 15  2   25-30                                     __________________________________________________________________________     .sup.(a) Method of Brunauer, Emmett and Teller, loc. cit.                     .sup.(b) Flat Plate Crush Strength, single pellet.                            .sup.(c) Determined by mercury intrusion to 55,000 psia using Micrometric     Autopore 9200 or 9210 (130° Contact angle, 0.473 N/m surface           tension of Hg).                                                               .sup.(e) Median pore diameter represents the pore diameter wherein 50% of     the total pore volume is found in pores having less than (or greater than     the median pore diameter.                                                

Pore size distribution curves measured by mercury intrusion techniquesnoted in footnote (c) above in Table 1 for carriers A-F are shown inFIGS. 2-7.

Of the carriers listed in Table 1, B and D are preferred because theyprovide catalysts which show better overall initial performance in termsof initial selectivity and initial activity.

Regardless of the character of the support or carrier used, it ispreferably shaped into particles, chunks, pieces, pellets, rings,spheres, wagon wheels and the like of a size suitable for employment infixed bed reactors. Conventional commercial fixed bed ethylene oxidereactors are typically in the form of a plurality of parallel elongatedtubes (in a suitable shell) approximately 0.7 to 2.7 inches O.D. and 0.5to 2.5 inches I.D. and 15-45 feet long filled with catalyst. In suchreactors, it is desirable to employ a support formed into a roundedshape, such as for example, spheres, pellets, rings, tablets and thelike, having diameters from about 0.1 inch to about 0.8 inch.

Particular supports may be selected having differing properties such assurface area and pore volume in order to provide particular catalyticproperties. With regard to surface area (B.E.T.) possible lower limitsare, for example, about 0.01, 0.03, 0.05, 0.075, 0.1, 0.15 and 0.2 m² /gand possible upper limits are, for example, about 0.6, 0.75, 0.9, 1, 2,2.5, 3, 4, 5 and 10 m² /g. With regard to water pore volume, possiblelower limits are, for example, about 0.05, 0.1, 0.15, 0.2 and 0.35 cc/gand possible upper limits are, for example, about 0.5, 0.55, 0.6, 0.65,0.7, 0.75 and 0.8 cc/g.

The catalysts of the instant invention are prepared by a technique inwhich the alkali metal promoters and the rhenium in the form of solublesalts and/or compounds are deposited on the catalyst and/or supportprior to, simultaneously with, or subsequent to the deposition of thesilver and each other. The alkali metal may be deposited at one step ofthe process and the rhenium at a different step or steps. The preferredmethod is to deposit silver, alkali metal and rhenium simultaneously onthe support, that is, in a single impregnation step, although it isbelieved that the individual or concurrent deposition of the alkalimetal and rhenium prior to and/or subsequent to the deposition of thesilver produces suitable catalysts.

Although alkali metals exist in a pure metallic state, they are not inthat form suitable for use. They are utilized as ions or salts orcompounds of alkali metals dissolved in a suitable solvent forimpregnation purposes. The porous carrier is impregnated with a solutionof alkali metal promoter ions, salt(s) and/or compound(s) before, duringor after impregnation or deposition of the silver ions, salt(s),complex(es) and/or compound(s) has taken place. An alkali metal promotermay even be deposited on the carrier after reduction to metallic silverhas taken place. The promoting amount of alkali metal utilized willdepend on several variables, such as, for example, the surface area andpore structure and surface chemical properties of the carrier used,silver content of the catalyst and the particular ions used inconjunction with the higher alkali metal cation or rhenium and amountsof rhenium present. The amount of alkali metal promoter deposited uponthe support or present on the catalyst generally lies between about 10and about 3000, preferably between about 15 and about 2000 and morepreferably between about 20 and about 1500 parts by weight per millionparts by weight of total catalyst. Most preferably, the amounts rangebetween about 50 and about 1000 parts per million by weight of the totalcatalyst. The degree of benefit obtained within the above-defined limitswill vary depending upon particular properties and characteristics, suchas, for example, reaction conditions, catalyst preparative techniques,surface area and pore structure and surface chemical properties of thecarrier utilized, silver content of the catalyst and other compounds,cations or anions present in addition to alkali metal ions such as theions added with the alkali metal or rhenium or compounds remaining fromthe impregnating solution, and the above-defined limits were selected tocover the widest possible variations in properties and characteristics.The effects of these variations are readily determined byexperimentation. The alkali metal promoters are present on the catalystsin the form of cations (ions) or compounds or complexes or surfacecompounds or surface complexes rather than as the extremely active freealkali metals, although for convenience purposes only in thisspecification and claims they are referred to as "alkali metal" or"alkali metal promoters", even though not present on the catalyst asmetals. For purposes of convenience the amount of alkali metal depositedon the support or present on the catalyst is measured as the metalrather than in the cationic or compound form. Thus, the alkali metalpromoters are present on the support or catalyst in the form of cations(ions) or compounds or complexes or surface compounds or surfacecomplexes. Without intending to limit the scope of the invention, it isbelieved that the alkali metal compounds are oxidic compounds. Moreparticularly, it is believed that the alkali metal compounds areprobably in the form of mixed surface oxides or double surface oxides orcomplex surface oxides with the aluminum of the support and/or thesilver of the catalyst, possibly in combination with species containedin or formed from the reaction mixture such as chlorides or carbonatesor residual species from the impregnation solution(s).

In a preferred embodiment, at least a major proportion (greater than50%) of the alkali metals is selected from the group consisting ofpotassium, rubidium, cesium and mixtures thereof.

In this preferred embodiment, the alkali metals comprise the higheralkali metals. As used herein the term "higher alkali metal" andcognates thereof refers to the alkali metals selected from the groupconsisting of potassium, rubidium, cesium and mixtures thereof. As usedherein, the term "mixtures of alkali metals" or "mixtures of higheralkali metals" or cognates of these terms refers to the use of two ormore of the alkali or higher alkali metals, as appropriate, to provide apromoting effect. Non-limiting examples include cesium plus rubidium,cesium plus potassium, cesium plus sodium, cesium plus lithium, cesiumplus rubidium plus sodium, cesium plus potassium plus sodium, cesiumplus lithium plus sodium, cesium plus rubidium plus potassium plussodium, cesium plus rubidium plus potassium plus lithium, cesium pluspotassium plus lithium and the like. When the alkali metal comprisesmixtures of higher alkali metals, at least two of the following areused, potassium, rubidium or cesium. Thus, for example, in the preferredembodiment wherein the higher alkali metal comprises potassium,rubidium, cesium or mixtures thereof, potassium may be used with cesium,or rubidium may be used with cesium, or potassium may be used withrubidium or all three may be used together. Hence, for example whenpotassium is used with cesium, the weight percent ration of potassium tocesium will range from 0/100 to 100/0, including all ranges in betweensuch as 20/80, 50/50, 75/25 etc., and similar relationships will applyto other mixtures. A particularly preferred alkali metal promoter iscesium.

It must be clear that the amounts of alkali metal promoters on thecatalysts are not necessarily the total amounts of these metals presentin the catalyst. They are amounts that have been added to the catalystby impregnation with suitable solutions of ions, salts and/or compoundsand/or complexes of alkali metals. These amounts do not include amountsof alkali metals that are locked into the support, say by calcining, orare not extractable in a suitable solvent such as water or lower alkanolor amine or mixtures thereof and do not provide a promoting effort. Itis also understood that the source of the alkali metal promoter ions,salts and/or compounds used to impregnate the catalyst may be thecarrier. That is, the carrier may contain extractable amounts of alkalimetal that can be extracted with a suitable solvent such as water orlower alkanol, thus preparing an impregnating solution from which thealkali metal ions, salts and/or compounds are deposited or redepositedon the support.

As used herein, the term "compound" refers to the combination of aparticular element with one or more different elements by surface and/orchemical bonding, such as ionic and/or covalent and/or coordinatebonding. The term "ionic" or "ion" refers to an electrically chargedchemical moiety; "cationic" or "cation" being positive and "anionic" or"anion" being negative. It is understood that ions do not exist invacuo, but are found in combination with charge-balancing counter ions.The term "oxidic" refers to a charged or neutral species wherein anelement in question is bound to oxygen and possibly one or moredifferent elements by surface and/or chemical bonding, such as ionicand/or covalent and/or coordinate bonding. Thus, an oxidic compound isan oxygen-containing compound which also may be a mixed, double orcomplex surface oxide. Illustrative oxidic compounds include, by way ofnonlimiting example, oxides (containing only oxygen as the secondelement), hydroxides, nitrates, sulfates, carboxylates, carbonates,bicarbonates, oxyhalides, etc., as well as surface species wherein theelement in question is bound directly or indirectly to an oxygen eitherin the substrate or on the surface.

As used herein, the term "promoting amount" of a certain component of acatalyst refers to an amount of that component that works effectively toprovide an improvement in one or more of the catalytic properties ofthat catalyst when compared to a catalyst not containing said component.Examples of catalytic properties include, inter alia, operability(resistance to runaway), selectivity, activity, conversion, stabilityand yield. It is understood by one skilled in the art that one or moreof the individual catalytic properties may be enhanced by the "promotingamount" while other catalytic properties may or may not be enhanced ormay even be diminished. It is further understood that differentcatalytic properties may be enhanced at different operating conditions.For example, a catalyst having enhanced selectivity at one set ofoperating conditions may be operated at a different set of conditionswherein the improvement shows up in the activity rather than theselectivity and an operator of an ethylene oxide plant willintentionally change the operating conditions in order to take advantageof certain catalytic properties even at the expense of other catalyticproperties in order to maximize profits by taking into account feedstock costs, energy costs, by-product removal costs and the like. Theparticular combination of silver, support, alkali metal and rhenium ofthe instant invention will provide an improvement in one or morecatalytic properties over the same combination of silver and support andnone or one of the promoters selected from rhenium and alkali metal.

As used herein, the term "catalytically effective amount of silver"refers to an amount of silver that provides a measurable conversion ofethylene and oxygen to ethylene oxide.

The carrier is also impregnated with rhenium ions, salt(s), compound(s)and/or complex(es). This may be done at the same time that the alkalimetal promoter is added, before or later; or at the same time that thesilver is added or before or later. Preferably rhenium, alkali metal andsilver are in the same impregnating solution, although it is believedthat their presence in different solutions will still provide suitablecatalysts. The preferred amount of rhenium, calculated as the metal,deposited on or present on the carrier or catalyst ranges from about 0.1mmoles to about 10 mmoles, more preferably from about 0.2 mmoles toabout 5 mmoles per kilogram of total catalyst, or alternatively statedfrom about 19 to about 1860, more preferably from about 37 to about 930parts by weight per million parts by weight of total catalyst. Thedegree of benefit obtained within the above-defined limits will varydepending upon particular properties and characteristics, such as, forexample, reaction conditions, catalyst preparative conditions, surfacearea and pore structure and surface chemical properties of the carrierutilized, silver and alkali content of the catalyst, and othercompounds, anions or cations present beside those containing rhenium oralkali metal, such as the ions added with the alkali metal or rhenium,or compounds remaining from the impregnation technique, and theabove-defined limits were selected to cover the widest possiblevariations in properties and characteristics. These variations arereadily determined by experimentation. For purposes of convenience, theamount of rhenium present on the catalyst is measured as the metal,irrespective of the form in which it is present.

The promoting effect provided by the rhenium can be affected by a numberof variables such as, for example, reaction conditions, catalystpreparative techniques, surface area and pore structure and surfacechemical properties of the support, the silver and alkali metal contentof the catalyst, the presence of other compounds, cations and anionspresent on the catalyst alone or in combination with the alkali metaland/or rhenium such as the ions added with the alkali metal or rhenium,or compounds remaining from the impregnating solution. The presence ofother activators, stabilizers, promoters, enhancers or other catalystimprovers can also affect the promoting effects of the rhenium. It isunderstood that any supported silver-based, alkali metal promotedethylene oxide catalyst which contains other cations and/or anions orany other activators, promoters, enhancers, stabilizers or othercatalyst improvers and which contains an amount of rhenium whichprovides a promoting effect, more preferably which provides higherethylene oxidation selectivities to ethylene oxide at a given oxygenconversion level and most preferably which provides higher initialethylene oxidation selectivities than is obtained under the samereaction conditions with the same catalyst not containing a promotingamount of rhenium will fall within the scope of the instant inventionand claims.

The rhenium compounds, salts and/or complexes used in the preparation ofthe instant catalysts are rhenium compounds, salts, and/or complexesthat can be solublized in an appropriate solvent. Preferably the solventis a water-containing solvent. More preferably the solvent is the samesolvent used to deposit the silver and the alkali metal promoter.Examples of rhenium compounds include the rhenium salts such as rheniumhalides, the rhenium oxyhalides, the rhenates, the perrhenates, theoxides and the acids of rhenium. A preferred compound to be utilized inthe impregnation solution is the perrhenate, preferably ammoniumperrhenate. However, the alkali metal perrhenates, alkaline earth metalperrhenates, silver perrhenate, other perrhenates and rhenium heptoxidecan also be suitably utilized. Rhenium heptoxide, Re₂ O₇, when dissolvedin water, hydrolyzes to perrhenic acid, HReO₄, or hydrogen perrhenate.Thus, for purposes of this specification rhenium heptoxide can beconsidered to be a perrhenate, i.e., ReO⁻ ₄. It is also understood thatthere are many rhenium compounds that are not soluble per se in water.However, these compounds can be solubilized by utilizing various acids,bases, peroxides, alcohols, etc. After solubilization these compoundscould be used, for example, with an appropriate amount of water or othersuitable solvent to impregnate the carrier. Of course, it is alsounderstood that upon solubilization of many of these compounds, theoriginal compound no longer exists after solubilization. For example,rhenium metal is not soluble in water. However, it is soluble inconcentrated nitric acid as well as in hydrogen peroxide solution. Thus,by using an appropriate reactive solvent one could use rhenium metal toprepare a solubilized rhenium-containing impregnating solution.

A presently preferred aspect of the instant invention is that therhenium present on the catalyst is present in a form that is extractablein a dilute aqueous base solution. For the purposes of thisspecification a 20 millimolar aqueous sodium hydroxide solution waschosen as the standard solution to be used to test the extractability ofrhenium on the catalyst. It will be clear to one skilled in the art thatother concentrations of sodium hydroxide as well as other bases can beutilized to test the extractability of rhenium. Thus, one skilled in theart can utilize other bases, for example, other alkali metal hydroxides,other alkaline earth metal hydroxides, ammonium hydroxide, organicbases, etc., suitably dissolved in an appropriate solvent to extractrhenium and by comparing it with the 20 millimolar aqueous sodiumhydroxide solution used herein can determine whether rheniumextractability with other base solutions will be equivalent to therhenium extractability with the 20 millimolar sodium hydroxide solution.

In the above-noted presently preferred embodiment, the rhenium is notpresent in the free metallic state, but rather is present as a compound,complex or ion. In a particularly preferred embodiment, the rhenium onthe catalyst is in a form that is extractable by dilute basic solution,and particularly with the 20 millimolar dilute sodium hydroxide solutiondisclosed herein. The base extraction technique can be used on a freshcatalyst, i.e., a catalyst that has gone through all the appropriatepreparative techniques and is ready to be placed in an ethylene oxidereactor, or on a used catalyst, i.e., a catalyst that has been used forthe production of ethylene oxide and then removed from the reactor. In atypical test procedure utilized herein a 1 to 10 g sample of fresh orreactor-tested catalyst is extracted with 10 to 50 milliters of the 20millimolar aqueous sodium hydroxide solution at 100° C. for 10 minutes.The amount of rhenium in an aliquot of the cooled extract is determinedspectrophotometrically following the procedure of V. W. Meloche et al.,Analytical Chemistry, 29, 527 (1957). In this procedure, a coloredrhenium complex with alpha-furildioxime is formed by reduction of therhenium species with tin (II) chloride in a dilute hydrochloric acidsolution containing a large excess of alpha-furildioxime.

Generally, the carrier is contacted with a silver salt, a silvercompound, or a silver complex which has been dissolved in an aqueoussolution, so that the carrier is impregnated with said aqueous solution,thereafter the impregnated carrier is separated from the aqueoussolution, e.g., by centrifugation or filtration and then dried. The thusobtained impregnated carrier is heated to reduce the silver to metallicsilver. It is conveniently heated to a temperature in the range from 50°C. to 600° C., during a period sufficient to cause reduction of thesilver salt, complex or compound to metallic silver and to form a layerof finely divided silver, which is bound to the surface of the carrier,both the exterior and pore surface. Air, or other oxygen containing gas,reducing gas, an inert gas or mixtures thereof may be conducted over thecarrier during this heating step.

There are several known methods to add the silver to the carrier orsupport. The carrier may be impregnated with an aqueous solutioncontaining silver nitrate dissolved therein, and then dried, after whichdrying step the silver nitrate is reduced with hydrogen or hydrazine.The carrier may also be impregnated with an ammoniacal solution ofsilver oxalate or silver carbonate, and then dried, after which dryingstep the silver oxalate or silver carbonate is reduced to metallicsilver by heating, e.g., to about 600° C. Specific solutions of silversalts with solubilizing and reducing agents may be employed as well,e.g., combinations of the vicinal alkanolamines, alkylenediamines andammonia.

One such example of a solution of silver salts comprises an impregnatingsolution comprising:

A. a silver salt of a carboxylic acid,

B. an organic amine alkaline solubilizing/reducing agent,

C. an aqueous solvent.

Suitable carboxylic acid silver salts include silver carbonate and thesilver salts of mono- and polybasic carboxylic and hydroxycarboxylicacids of up to about 16 carbon atoms. Silver carbonate and silveroxalate are particularly useful silver salts, with silver oxalate beingmost preferred.

An organic amine solubilizing/reducing agent is present in theimpregnating solution. Suitable organic amine silversolubilizing/reducing agents include lower alkylenediamines of from 1 to5 carbon atoms, mixtures of a lower alkylenediamine of from 1 to 5carbon atoms with a lower alkylenediamine of from 1 to 5 carbon atoms,as well as mixtures of ammonia with lower alkanolamines or loweralkylenediamines of from 1 to 5 carbons. Four groups of organic aminesolubilizing/reducing agents are particularly useful. They are thefollowing:

A. vicinal alkylenediamines of from 2 to 4 carbon atoms;

B. mixtures of (1) vicinal alkanolamines of from 2 to 4 carbon atoms and(2) vicinal alkylenediamines of from 2 to 4carbon atoms;

C. mixtures of vicinal alkylenediamines of from 2 to 4 carbon atoms andammonia; and

D. mixtures of vicinal alkanolamines of from 2 to 4 carbon atoms andammonia. These solubilizing/reducing agents are generally added in theamount of from 0.1 to 10 moles per mole of silver present.

Particularly preferred solubilizing/reducing agents are:

A. ethylenediamine,

B. ethylenediamine in combination with ethanolamine,

C. ethylenediamine in combination with ammonia, and

D. ethanolamine in combination with ammonia.

Ethylenediamine is most preferred. Ethylenediamine in combination withethanolamine gives comparable results, but it is believed thatimpurities that are present in certain commercially availableethanolamine preparations can produce inconsistent results.

When ethylenediamine is used as the sole solubilizing/reducing agent, itis necessary to add amounts of the amine in the range of from 0.1 to 5.0moles of ethylenediamine per mole of silver.

When ethylenediamine and ethanolamine together are used as thesolubilizing/reducing agent, it is suitable to employ from 0.1 to 3.0moles of ethylenediamine per mole of silver and from 0.1 to 2.0 moles ofethanolamine per mole of silver.

When ethylenediamine or ethanolamine is used with ammonia, it isgenerally useful to add at least about two moles of ammonia per mole ofsilver and very suitable to add from about 2 to about 10 moles ofammonia per mole of silver. The amount of ethylenediamine orethanolamine employed then is suitably from 0.1 to 2.0 moles per mole ofsilver.

One method of preparing the silver-containing catalyst can be found inU.S. Pat. No. 3,702,259, issued Nov. 7, 1972, incorporated by referenceherein. Other methods for preparing the silver-containing catalystswhich in addition contain higher alkali metal promoters can be found inU.S. Pat. No. 4,010,115 issued Mar. 1, 1977; U.S. Pat. No. 4,356,312,issued Oct. 26, 1982; U.S. Pat. No. 3,962,136, issued June 8, 1976 andU.S. Pat. No. 4,012,425, issued Mar. 15, 1977 all incorporated byreference herein.

The preferred amount of alkali metal promoter deposited on or present onthe surface of the carrier or catalyst generally lies between about 10and about 3000, preferably between about 15 and about 2000 and morepreferably between about 20 and about 1500 ppm by weight of alkali metalcalculated on the total catalyst material. Amounts between about 50 andabout 1000 ppm are most preferable. Suitable compounds of alkali metalare, for example, the nitrates, carbonates, bicarbonates, oxalates,carboxylic acid salts or hydroxides put in solution, preferably aqueoussolution. The more preferred promoters among the alkali metals are thealkali metals comprising the higher alkali metals comprising potassium,rubidium, cesium or mixtures thereof in a promoting amount with the evenmore preferred promoters being rubidium and/or cesium. Preferably theamount ranges from about 10 and about 3000, more preferably betweenabout 15 and about 2000, even more preferably between about 20 and about1500 ppm by weight, and most preferably between about 50 and 1000 ppm byweight. The most preferred promoter is cesium, preferably applied as anaqueous solution having cesium nitrate or cesium hydroxide dissolvedtherein. While the higher alkali metals provide the most significanteffect when considering the selectivity, particularly the initialselectivity, it is considered within the scope of the instant preferredembodiment that lithium and/or sodium may also be present in addition tothe higher alkali metal(s) in order to provide enhanced or differenteffects. Thus, the use of Markush terminology in this specification andclaims to indicate the higher alkali metals cesium and/or rubidiumand/or potassium is not meant and does not exclude the presence,inclusion or the use of lithium and/or sodium in addition to the higheralkali metals. Thus, the use of a Markush recitation in the instantspecification and claims means that the elements in the recitation areincluded, but others are not excluded, i.e., the Markush recitation isan open ended recitation.

There are known excellent methods of applying the promoterscoincidentally with the silver on the carrier. Suitable alkali metalsalts are generally those which are soluble in the silver-impregnatingliquid phase. Besides the above-mentioned compounds may be mentioned thenitrites; the halides, such as fluorides, chlorides, iodides, bromides;oxyhalides; bicarbonates; borates; sulfates; sulfites; bisulfates;acetates; tartrates; lactates and isopropoxides, etc. The use of rheniumor alkali metal salts which have ions which react with the silver ionsin solution is preferably avoided, e.g. the use of cesium chloridetogether with silver nitrate in an aqueous solution, since then somesilver chloride is prematurely precipitated. Here the use of cesiumnitrate is recommended instead of cesium chloride, for example. However,cesium chloride may be used together with a silver salt-amine-complex inaqueous solution, since then the silver chloride is not precipitatedprematurely from the solution.

The promoters may be deposited on the carrier (support) or on thecatalyst depending upon the particular impregnation technique orsequence utilized. As used in this specification and claims, the term"on the catalyst" when referring to the deposition or presence ofpromoters and/or co-promoters refers to the catalyst which comprises thecombination of carrier (support) and silver. Thus, the promoters, i.e.,alkali metal and rhenium may be found individually or in a mixturethereof on the catalyst, on the support or on both the catalyst and thesupport. There may be, for example, alkali and rhenium on the catalyst;alkali and rhenium on the support; alkali on the support and rhenium onthe catalyst; alkali on the support and a mixture of alkali and rheniumon the catalyst; rhenium on the support and a mixture of alkali andrhenium on the catalyst; rhenium on the support and alkali on thecatalyst; a mixture of alkali and rhenium on the support and a mixtureof alkali and rhenium on the catalyst; a mixture of alkali and rheniumon the support and alkali on the catalyst; and a mixture of alkali andrhenium on the support and rhenium on the catalyst.

The amount of the alkali metal and/or rhenium promoters on the porouscarrier or catalyst may also be regulated within certain limits bywashing out the surplus of promoter material with an appropriatesolvent, for example, methanol or ethanol.

A particularly preferred process of impregnating the carrier consists ofimpregnating the carrier with an aqueous solution containing a silversalt of a carboxylic acid, an organic amine, a salt of cesium andammonium perrhenate dissolved therein. Silver oxalate is a preferredsilver salt. It can be prepared by reacting silver oxide (slurry inwater) with (a) a mixture of ethylenediamine and oxalic acid, or (b)oxalic acid and then ethylenediamine, which latter is preferred, so thatan aqueous solution of silver oxalate-ethylenediamine complex isobtained, to which solution is added a certain amount of cesium compoundand ammonium perrhenate. While addition of the amine to the silver oxidebefore adding the oxalic acid is possible, it is not preferred since itcan give rise to solutions which are unstable or even explosive innature. Other diamines and other amines, such as ethanolamine, may beadded as well. A cesium-containing silver oxalate solution may also beprepared by precipitating silver oxalate from a solution of cesiumoxalate and silver nitrate and rinsing with water or alcohol theobtained silver oxalate in order to remove the adhering cesium saltuntil the desired cesium content is obtained. The cesium-containingsilver oxalate is then solubilized with ammonia and/or an amine in waterand ammonium perrhenate is added. Rubidium-, potassium-, sodium-,lithium- and mixtures of alkali metal-containing solutions may beprepared also in these ways. The impregnated carriers are then heated toa temperature between 50° C. and 600° C., preferably between 75° C. and400° C. to evaporate the liquid and produce a metallic silver.

In general terms, the impregnation process comprises impregnating thesupport with one or more solutions comprising silver, alkali metal andrhenium. As used in the instant specification and claims, theterminology "impregnating the support with one or more solutionscomprising silver, alkali metal and/or rhenium" and similar or cognateterminology means that the support is impregnated in a single ormultiple impregnation, with one solution containing silver, alkali metaland rhenium; in multiple impregnations with two or more solutionscontaining silver, alkali metal and rhenium in differing amounts; or inmultiple impregnations with two or more solutions, wherein each solutioncontains at least one component selected from silver, alkali metal andrhenium with the proviso that all of the components silver, alkali metaland rhenium will individually be found in at least one of the solutions.The concentration of the silver (measured as the metal) in thesilver-containing solution will range from about 1 g/liter up to thesolubility limit of silver in the solution, preferably from about 10 g/lup to the solubility limit when a single impregnation is utilized. Theconcentration of the alkali metal (measured as the metal) will rangefrom about 1×10⁻³ g/liter to about 12 g/l when a single impregnation isutilized. The concentration of the rhenium (measured as the metal) willrange from about 5×10⁻³ g/l to about 20 g/l, preferably from about50×10⁻³ g/l to about 20 g/l when a single impregnation step is utilized.Concentrations selected within the above-noted ranges will depend uponthe pore volume of the catalyst, the final amount desired in the finalcatalyst and whether the impregnation is single or multiple. Appropriateconcentrations can readily be determined by routine experimentation.

The amount of silver deposited on the support or present on the supportis to be a catalytically effective amount of silver, i.e., an amountthat catalyzes the reaction of ethylene and oxygen to produce ethyleneoxide. Preferably this amount will range from about 1 to about 30percent by weight of the total catalyst, more preferably from about 1 toabout 25 percent by weight of the total catalyst, and even morepreferably from about 5 to about 20 percent by weight of the totalcatalyst. The upper and lower limit of preferred silver concentrationscan be suitably varied, depending upon the particular catalyticproperties or effect desired or other variables involved. Possible lowerlimits of silver are, for example, about 1, 3, 5, 6, 8 and 10 percent byweight of the total catalyst. Possible upper limits of silver are, forexample, about 15, 16, 18, 20, 22, 25 and 30 percent by weight of thetotal catalyst.

The amount of alkali metal deposited on the support or catalyst orpresent on the support or catalyst is to be a promoting amount.Preferably the amount will range from about 10 to about 3000, morepreferably from about 15 to about 2000, even more preferably from about20 to about 1500 and yet even more preferably from about 50 to about1000 ppm by weight of the total catalyst, measured as the metal. Theupper and lower limits of preferred alkali metal concentrations can besuitably varied depending upon the particular promoting effect desiredor other variables involved. Possible lower limits of alkali metals are,for example, about 1, 5, 10, 25, 50, 75, 100, 200 and 300 ppm by weightof the total catalyst, measured in the metal. Possible upper limits ofalkali metal are, for example, about 250, 300, 400, 500, 600, 700, 800,900, 1000, 1250, 1500, 2000, 2500 and 3000 ppm by weight of the totalcatalyst, measured as the metal.

The amount of rhenium deposited on the support or catalyst or present onthe support or catalyst is to be a promoting amount. Preferably theamount will range from about 0.01 to about 15, more preferably fromabout 0.1 to about 10, even more preferably from about 0.2 to about 5and yet even more preferably from about 0.5 to about 4 μmoles/g of totalcatalyst, measured as the metal. The upper and lower limits of preferredrhenium concentrations can be suitably varied depending upon theparticular promoting effect desired or other variables involved.Possible lower limits of rhenium are, for example, about 0.01, 0.1, 0.2,0.25, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 and 1.5 μmoles/g of totalcatalyst. Possible upper limits of rhenium are, for example, about 2.5,3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 15 and 16 μmoles/g of total catalyst.

It is observed that independent of the form in which the silver ispresent in the solution before precipitation on the carrier, the term"reduction to metallic silver" is used, while in the meantime oftendecomposition by heating occurs. It is preferred to use the term"reduction", since the positively charged Ag⁺ ion is converted intometallic Ag atom. Reduction times may generally vary from about 0.5minute to about 8 hours, depending on the circumstances.

The silver catalysts according to the present invention have been shownto have a particularly high initial selectivity for ethylene oxide inthe direct oxidation of ethylene with molecular oxygen to ethyleneoxide. The conditions for carrying out such an oxidation reaction in thepresence of the silver catalysts according to the present inventionbroadly comprise those already described in the prior art. This applies,for example, to suitable temperatures, pressures, residence times,diluent materials, such as nitrogen, carbon dioxide, steam, argon,methane or other saturated hydrocarbons, presence of moderating agentsto control the catalytic action, for example, 1-2-dichloroethane, vinylchloride or chlorinated polyphenyl compounds, the desirability ofemploying recycle operations or applying successive conversions indifferent reactors to increase the yields of ethylene oxide, and anyother special conditions which may be selected in processes forpreparing ethylene oxide. Pressures in the range of from atmospheric to35 bar are generally employed. Higher pressures are, however, by nomeans excluded. Molecular oxygen employed as reactant can be obtainedfrom conventional sources. The suitable oxygen charge may consistessentially of relatively pure oxygen, a concentrated oxygen streamcomprising oxygen in major amount with lesser amounts of one or morediluents, such as nitrogen and argon, or another oxygen-containingstream, such as air. It is therefore evident that the use of the presentsilver catalysts in ethylene oxidation reactions is in no way limited tothe use of specific conditions among those which are known to beeffective. For purposes of illustration only, the following table showsthe range of conditions that are often used in current commercialethylene oxide reactor units.

                  TABLE 2                                                         ______________________________________                                        *GHSV                1500-10,000                                              Inlet pressure       150-400 psig                                             Inlet Feed                                                                    ethylene             1-40%                                                    O.sub.2              3-12%                                                    CO.sub.2             2-40%                                                    ethane               0-3%                                                     Argon and/or methane and/or nitrogen diluent                                  chlorohydrocarbon moderator 0.3-20 ppmv total                                 Coolant temperature  180-315° C.                                       Catalyst temperature 180-325° C.                                       O.sub.2 conversion level                                                                           10-60%                                                   EO Production (Work Rate)                                                                          2-16 lbs. EO/cu.                                                              ft. catalyst/hr.                                         ______________________________________                                         *Liters of gas at standard temperature and pressure passing over the one      liter of packed catalyst per hour.                                       

In a preferred application of the silver catalysts according to thepresent invention, ethylene oxide is produced when an oxygen-containinggas is contacted with ethylene in the presence of the present catalystsat a temperature in the range of from about 180° C. to about 330° C. andpreferably about 200° C. to about 325° C.

The invention will be illustrated by the following illustrativeembodiments which are provided for illustration only and are notintended to limit the scope of the instant invention.

ILLUSTRATIVE EMBODIMENTS Illustrative Embodiment 1

The following illustrative embodiment describes typical preparativetechniques for making the catalysts of the instant invention (andcomparative catalysts) and the typical technique for measuring theproperties of these catalysts.

Part A: Preparation of stock silver oxalate/ethylenediamine solution foruse in catalyst preparation

(1) Dissolve 415 g reagent-grade NaOH in 2340 ml deionized water. Adjusttemperature to 50° C.

(2) Dissolve 1699 g "spectropure" (high-purity) AgNO₃ in 2100 mldeionized water. Adjust temperature to 50° C.

(3) Add NaOH solution slowly to AgNO₃ solution with stirring,maintaining temperature at 50° C. Stir for 15 minutes after addition iscomplete, then lower temperature to 40° C.

(4) Insert clean filter wands and withdraw as much water as possiblefrom the precipitate created in step (3) in order remove sodium andnitrate ions. Measure the conductivity of the water removed and add backas much fresh deionized water as was removed by the filter wands. Stirfor 15 minutes at 40° C. Repeat this process until the conductivity ofthe water removed is less than 90 μmho/cm. Then add back 1500 mldeionized water.

(5) Add 630 g of high-purity oxalic acid dihydrate in approximately 100g increments. Keep the temperature at 40° C. and stir to mix thoroughly.Add the last portion of oxalic acid dihydrate slowly and monitor pH toensure that pH does not drop below 7.8. Aim for a pH endpoint of8.0-8.4. Add high-purity silver oxide if necessary to achieve thisendpoint.

(6) Remove as much water from the mixture as possible using clean filterwands in order to form a highly concentrated silver-containing slurry.Cool the silver oxalate slurry to 30° C.

(7) Add 699 g of 92% w ethylenediamine (8% deionized water). Do notallow the temperature to exceed 30° C. during addition.

The above procedure yields a solution containing approximately 27-33% wAg.

Part B: Catalyst Impregnation Procedures

Catalyst support Example B described in Table 1 is a preferred supportfor the instant invention and is used in the following examples andillustrative embodiments.

Preparation of undoped impregnating solution is as follows: The stock Agoxalate/ethylenediamine solution of Part A is diluted preferably withdeionized water, or alternatively may be diluted with monoethanolamine,or a mixture of deionized water and monoethanolamine to achieve a Agconcentration of approximately 27.6% by weight. The use ofmonoethanolamine or monoethanolamine plus water to dilute the stocksolution is believed to provide catalysts comparable to those obtainedby the use of water. However, it is believed that certain impuritiespresent in monoethanolamine can cause inconsistent results in thecatalysts made with monoethanolamine. Hence, water is preferred and wasused for all of the examples provided herein.

Preparation of doped impregnation solution is as follows:

For catalyst A (Cs only): Add 46.4 mg of aqueous CsOH solution (50.7% wCs) directly to 50 g of undoped impregnating solution.

For catalyst B (Cs-Re): Dissolve 55.0 mg of NH₄ ReO₄ in a minimum volumeof 50/50 (w/w) ethylenediamine/deionized water and add to 50 g ofundoped impregnation solution. Then add 84.7 mg of aqueous CsOH solution(50.7% w Cs) to the same impregnating solution.

The aqueous cesium hydroxide solution used for catalyst preparation inthis and the following illustrative embodiments was doped with aradioactive isotope of cesium (¹³⁴ Cs) so that the cesium levels on thefinished catalysts could be readily determined by radiotracer analysis.(Alternatively, the levels of cesium and other alkali promoters onfinished catalysts can be determined by the water leaching methoddescribed below.) The concentration of cesium in this aqueous,radiolabeled cesium hydroxide solution was determined to be 50.7% w byneutron activation analysis at the Nuclear Science Center, Texas A&MUniversity, College Station, Texas, using a TRIGA reactor, an Ortechigh-purity Germanium Model BA-GEM-25185 detector, and a Tracor NorthernModel 4000 multichannel analyzer. All target and actual cesium levelsreported for catalysts in this and the following illustrativeembodiments are based upon a value of 50.7% w for the concentration ofcesium in the stock, radiolabeled cesium hydroxide solution. However,when this same cesium hydroxide solution was subsequently analyzed byinductively coupled plasma jet-mass spectrometry using a SCIEX Elan 250instrument, the cesium concentration was found to be 45% w. If thislatter value for the cesium concentration in this solution is closer tothe actual value, then the absolute levels of cesium for the catalystsdescribed in this and the following illustrative embodiments would beapproximately 11.2% lower than those reported.

Part C: Catalyst impregnation and curing

Approximately 30 g of carrier B are placed under 25 mm vacuum for 3minutes at room temperature. Approximately 50 g of doped impregnatingsolution is then introduced to submerge the carrier, and the vacuum ismaintained at 25 mm for an additional 3 minutes. At the end of thistime, the vacuum is released, and excess impregnating solution isremoved from the carrier by centrifugation for 2 minutes at 500 rpm. Ifthe impregnating solution is prepared without monoethanolamine, then theimpregnated carrier is then cured by being continuously shaken in a 300cu. ft./hr. air stream flowing across a cross-sectional area ofapproximately 3-5 square inches at 250° C. for 5 minutes. If significantmonoethanolamine is present in the impregnating solution, then theimpregnated carrier is cured by being continuously shaken in a 300 cu.ft./hr. air stream at 250° C. for 2.5 minutes, followed by a 100 cu.ft./hr. air stream at 270° C. for 7.5 minutes (all over across-sectional area of approximately 3-5 square inches). The curedcatalyst is then ready for testing.

This procedure will yield catalysts on this carrier containingapproximately 13.5% w Ag with the following approximate dopant levelsand which are approximately optimum in cesium for the given silver andrhenium levels and support with regard to initial selectivity under thetest conditions described below.

    ______________________________________                                        catalyst      Cs, ppmw  Re, ppmw                                              ______________________________________                                        A             230        0                                                    B             420       372                                                   ______________________________________                                    

The actual silver content of the catalyst can be determined by any of anumber of standard, published procedures. The actual level of rhenium onthe catalysts prepared by the above process can be determined byextraction with 20 mM aqueous sodium hydroxide solution, followed byspectrophotometric determination of the rhenium in the extract, asdescribed above. The actual level of cesium on the catalyst can bedetermined by employing a stock cesium hydroxide solution, which hasbeen labeled with a radioactive isotope of cesium, in catalystpreparation. The cesium content of the catalyst can then be determinedby measuring the radioactivity of the catalyst. Alternatively, thecesium content of the catalyst can be determined by leaching thecatalyst with boiling deionized water. In this extraction processcesium, as well as other alkali metals, is measured by extraction fromthe catalyst by boiling 10 grams of whole catalyst in 20 milliliters ofwater for 5 minutes, repeating the above two more times, combining theabove extractions and determining the amount of alkali metal present bycomparison to standard solutions of reference alkali metals using atomicabsorption spectroscopy (using Varian Techtron Model 1200 orequivalent). It should be noted that the cesium content of the catalystas determined by the water leaching technique may be lower than thecesium content of the catalyst as determined by the radiotracertechnique.

Part D: Standard Microreactor Catalyst Test Conditions/Procedure

3 to 5 g of crushed catalyst (14-20 mesh) are loaded into a 1/4 inchdiameter stainless steel U-shaped tube. The U tube is immersed in amolten metal bath (heat medium) and the ends are connected to a gas flowsystem. The weight of catalyst used and the inlet gas flow rate areadjusted to achieve a gas hourly space velocity of 3300 cc of gas per ccof catalyst per hour. The inlet gas pressure is 210 psig.

The gas mixture passed through the catalyst bed (in once-throughoperation) during the entire test run (including startup) consists of30% ethylene, 8.5% oxygen, 7% carbon dioxide, 54.5% nitrogen, and 4.4 to5.6 ppmv vinyl chloride.

The initial reactor (heat medium) temperature is 180° C. After 1 hour atthis initial temperature, the temperature is increased to 190° C. for 1hour, followed by 200° C. (1 hour), 210° C. (1 hour), 220° C. (1 hour),227° C. (2 hours), and 235° C. (2 hours), and 242° C. (2 hours). Thetemperature is then adjusted so as to achieve a constant oxygenconversion level of 40%. Performance data at this conversion level areusually obtained when the catalyst has been onstream for a total of 16±4hours and are referred to as "initial performance data" in the examplesgiven below. Due to slight differences in feed gas composition, gas flowrates, and the calibration of analytical instruments used to determinethe feed and product gas compositions, the measured selectivity andactivity of a given catalyst may vary slightly from one test run to thenext. To allow meaningful comparison of the preformances of catalyststested at different times, all catalysts described in this and thefollowing illustrative embodiments were tested simultaneously with astandard catalyst having the composition of catalyst A or with adifferent catalyst which has been standardized with reference tocatalyst A. All performance data reported in this and the followingillustrative embodiments are corrected and stated relative to theaverage initial performance of catalyst A (S₄₀ =80.0%; T₄₀ =242° C.).

Typical initial performances at 40% O₂ conversion for the above recipeare as follows:

    ______________________________________                                        catalyst                                                                             A      selectivity =                                                                            80.0% temperature =                                                                           242° C.                              B                 81.9%           248° C.                       ______________________________________                                    

Illustrative Embodiment 2

Using the general preparative technique of Illustrative Embodiment 1, aseries of catalysts were prepared utilizing carrier B described inTable 1. The catalysts were prepared without using monoethanolamine. Oneseries of catalysts contained 2 mmol (millimoles) of rhenium perkilogram of catalyst and the other series of catalysts was made in theidentical fashion except that they contained no rhenium. In both seriesthe concentration of cesium in the individual catalysts was varied. Thecatalysts were tested as described in Illustrative Embodiment 1 and theresults are shown in Table 3. The cesium levels reported in Table 3 wereobtained by the radiotracer analysis technique described in IllustrativeEmbodiment 1, assuming a concentration of 50.7%w cesium for theradiolabeled, aqueous cesium hydroxide solution used in catalystpreparation. Further, the results from these tests in the form of theinitial selectivity versus cesium concentration are plotted in FIG. 1.In this Figure one can see the beneficial effects of rhenium which areindicated by the highlighted area between the two curves to the right oftheir cross-over point. It can be seen from FIG. 1 that the use ofrhenium provides not only an increase in the absolute value of theinitial selectivity obtained at optimum cesium concentration, but also asignificantly improved initial selectivity of the catalyst at highcesium concentrations e.g., 300 ppm cesium and over.

Illustrative Embodiment 3

A series of catalysts were prepared in a fashion similar to thetechnique described in Illustrative Embodiment 1 using differentcarriers having those properties described in Table 1 in thespecification. The catalysts were made without monethanolamine. Thecatalysts were tested as described in Illustrative Embodiment 1 and theresults are shown below in Table 4. Unless otherwise noted, allcatalysts listed in Table 4 have cesium levels which give the optimum(highest) initial selectivity obtained under these test conditions for acatalyst made on the indicated carrier with the indicated levels ofsilver and rhenium. The cesium levels reported in Table 4 were obtainedby the radiotracer analysis technique described in IllustrativeEmbodiment 1, assuming a concentration of 50.7% w cesium for theradiolabeled, aqueous cesium hydroxide solution used in catalystpreparation. Catalyst 4-6 was not made using the identical support ofcatalyst 4-5 but rather used a comparable support from a different lotwhich had a surface area of 0.44 m² /g, a water pore volume of 0.42cc/g, a water-leachable sodium content approximately 50% higher and anacid-leachable sodium content approximately 100% higher. (This supportis referred to hereinafter as C').

Illustrative Embodiment 4

A series of catalysts were prepared on carrier Example B of Table 1 in afashion similar to that described in Illustrative Embodiment 1 bututilizing different rhenium concentrations. The catalysts were madewithout monethanolamine. The catalysts were tested as described inIllustrative Embodiment 1 and the results are shown in Table 5 below.Unless otherwise noted, all catalysts listed in Table 5 have cesiumlevels which give the optimum (highest) initial selectivity obtainedunder these test conditions for a catalyst made on this support with theindicated levels of silver and rhenium. The cesium levels reported inTable 5 were obtained by the radiotracer analysis technique described inIllustrative Embodiment 1, assuming a concentration of 50.7% w cesiumfor the radiolabeled, aqueous cesium hydroxide solution used in catalystpreparation.

Illustrative Embodiment 5

A series of catalysts were prepared in a fashion similar to thatdescribed in Illustrative Embodiment 1 using support Example B ofTable 1. The catalysts were made without monoethanolamine. In thisseries different alkali metals were utilized as alkali metal hydroxides.The catalysts were tested as described in Illustrative Embodiment 1 andthe results are shown in Table 6 below. The alkali levels presentedrepresent target levels. Unless otherwise noted, all catalysts listed inTable 6 have target alkali levels which give the optimum (highest)initial selectivity obtained under these test conditions for a catalystmade with the indicated alkali metal hydroxide on this support with theindicated levels of silver and rhenium. For examples 6-13 and 6-, thetarget cesium content was fixed at 160 ppm and the rubidiumconcentration was optimized to provide the highest initial selectivityunder these test conditions at the indicated levels of silver andrhenium. Also, for these two examples, the support, which was otherwisecomparable to support B, had a surface of 0.45 m² /g instead of 0.42 m²/g and about 10-15% lower levels of leachable sodium (this support isreferred to hereinafter as support B').

Illustrative Embodiment 6

Two sets of catalysts were prepared in a fashion similar to thatdescribed in Illustrative Embodiment 1 using support Example B of Table1 with the exception that ammonium molybdate ((NH₄)₆ Mo₇ O₂₄.4H₂ O) wasadded to the impregnation solution in sufficient quantity to provideabout 96 ppm by weight of Mo in the final catalyst. The catalysts weremade without monoethanolamine. The catalysts contained potassium(target) levels which provide the optimum (highest) initial selectivityunder the test conditions described in Illustrative Embodiment 1 at thenoted levels of silver, rhenium and molybdenum. Catalyst example 7-1(prepared using support B') which contained 13.2% w silver, no rhenium,180 ppm K (target level) and 96 ppm Mo had an initial S₄₀ of 77.0% andan initial T₄₀ of 261° C. and catalyst example 7-2 which 14.5% w silver,186 ppm by weight of rhenium (target level), 160 ppm K (target level)and 96 ppm Mo (target level) had an initial S₄₀ of 81.1% and an initialT₄₀ of 279° C. For comparative purposes, note that catalyst Example 6-7which contains no rhenium or molybdenum has a S₄₀ of 79.4 and a T₄₀ of240° C.

Illustrative Embodiment 7

Two catalysts were prepared in a fashion similar to that described inIllustrative Embodiment 1 using support Example B of Table 1. Thecatalysts both contained cesium levels which had been optimized toprovided the highest initial selectivities under the test conditionsdescribed in Illustrative Embodiments 1 . Both catalysts were madewithout using monoethanolamine.

Catalyst VII-1 was prepared using 2 μmoles/g each of NH₄ ReO₄ and (NH₄)₂SO₄. Catalyst VII-2 was prepared using 2 μmoles/g each of (NH₄)ReO₄ andNa₂ SO₄. The catalysts were tested as described in IllustrativeEmbodiment 1 and the results are listed below:

    ______________________________________                                                        Cs*     Na**  Re**                                            Catalyst                                                                             % w Ag   ppmw    ppmw  umoles/g                                                                             S.sub.40                                                                            T.sub.40                           ______________________________________                                        VII-1  12.8     513      0    2      81.7% 274° C.                     VII-2  13.5     424     92    2      83.9% 253° C.                     ______________________________________                                         *by radiotracer, assuming a concentration of 50.7% w cesium for the           radiolabeled, aqueous cesium hydroxide solution used in catalyst              preparation.                                                                  **target levels                                                          

It can be seen from the above results that the catalyst containing themixture of cesium and sodium as alkali metal promoters is more selectiveand more active than the catalyst which contains only cesium as thealkali metal promoter.

Illustrative Embodiment 8

Three catalysts were prepared in a fashion similar to that described inIllustrative Embodiment 1 (no monoethanolamine) using support Example Bof Table 1. The catalysts contained cesium levels which had beenoptimized to provide the highest initial selectivities under the testconditions described in Illustrative Embodiment 1. Catalyst VIII-1 wasprepared using one μmole/g of NH₄ ReO₄ and two μmoles/g of (NH₄)₂ SO₄.Catalyst VIII-2 was prepared using one μmole/g of NH₄ ReO₄ and twoμmoles/g of Li₂ SO₄. Catalyst VIII-3 was prepared using one μmole/g ofNH₄ ReO₄ and two μmoles/g of Na₂ SO₄. The catalysts were tested asdescribed in Illustrative Embodiment 1 and the results are listed below:

    __________________________________________________________________________    Catalyst                                                                           % w Ag                                                                             Cs* ppmw                                                                            Li* ppmw                                                                            Na* ppmw                                                                            Re μmoles/g                                                                       S.sub.40                                                                          T.sub.40                               __________________________________________________________________________    VIII-1                                                                             13.8 505   0     0     1      82.0%                                                                             273° C.                         VIII-2                                                                             13.9 398   28    0     1      83.1%                                                                             249° C.                         VIII-3                                                                             14.8 411   0     92    1      80.9%                                                                             248° C.                         __________________________________________________________________________     *by radiotracer, assuming a concentration of 50.7% w cesium for the           radiolabeled, aqueous cesium hyroxide solution used in catalyst               preparation.                                                                  **target levels                                                          

It can be seen from the above results that Catalyst VIII-2 containing amixture of cesium plus lithium as the alkali promoters is more selectiveand more active than the catalyst with only cesium as the alkalipromoter. At this level of rhenium (half of that of IllustrativeEmbodiment 7), Catalyst VIII-3 containing both cesium plus sodium asalkali promoters shows improved activity over the comparable catalystcontaining only cesium as alkali promoter whereas the selectivity isdiminished.

                  TABLE 3                                                         ______________________________________                                        CS OPTIMIZATION FOR                                                           CATALYSTS WITH AND WITHOUT RE                                                                Re Target Cs, ppmw                                             Experiment                                                                            %      Level,    (Radiotracer                                                                           Initial                                                                             Initial                               No.     wAg    ppmw      Analysis)                                                                              S.sub.40, %                                                                         T.sub.40, °C.                  ______________________________________                                         3-1**  13.6    0         0       74.6  229                                    3-2**  13.6    0        104      77.6  232                                   3-3     14.3    0        236      80.0  242                                   3-4     14.3    0        301      79.4  243                                    3-5**  13.6    0        416      77.0  259                                   3-6     14.3    372*      0       54.3  236                                   3-7     14.3   372       110      69.9  243                                   3-8     14.3   372       209      75.8  239                                   3-9     14.3   372       327      79.8  240                                    3-10   14.2   372       403      81.8  245                                    3-11   14.2   372       438      81.9  248                                    3-12   14.2   372       488      81.4  250                                    3-13   14.2   372       512      81.0  251                                    3-14   14.2   372       561      80.3  256                                   ______________________________________                                         *2.0 μmoles/g                                                              **Performance data obtained at 40% oxygen conversion when catalyst had        been onstream for 32 ± 4 hours.                                       

                  TABLE 4                                                         ______________________________________                                        CESIUM OPTIMIZED CATALYSTS ON DIFFERENT                                       CARRIERS WITH AND WITHOUT RHENIUM                                                                  Re                                                       Experi-              Target                                                                              Cs, ppmw                                           ment          %      Level,                                                                              (Radiotracer                                                                           Initial                                                                             Initial                             No.   Carrier wAg    ppmw  Analysis)                                                                              S.sub.40, %                                                                         T.sub.40, °C.                ______________________________________                                        4-1   A       10.3   0     162      80.1  248                                 4-2   A       10.3   186*  261      81.4  253                                 4-3   B       14.3   0     236      80.0  242                                 4-4   B       14.2   372** 438      81.9  248                                 4-5   C       14.1   0     256      80.3  240                                 4-6    C'     13.5   372   469      81.4  241                                 4-7   D       15.0   0     309      80.9  240                                 4-8   D       15.1   372   434      82.7  249                                 ______________________________________                                         *1.0 μmoles/g                                                              **2.0 μmoles/g                                                        

                  TABLE 5                                                         ______________________________________                                        CS OPTIMIZED                                                                  CATALYSTS WITH DIFFERENT RE LEVELS                                                           Re Target                                                      Experi-        Level     Cs, ppmw                                             ment           ppmw      (Radiotracer                                                                           Initial                                                                             Initial                               No.   % wAg    (μmoles/g)                                                                           Analysis)                                                                              S.sub.40, %                                                                         T.sub.40, °C.                  ______________________________________                                        5-1   14.3      0(0)     236      80.0  242                                   5-2   13.8      93(0.5)  297      80.4  244                                   5-3   13.9     186(1.0)  360      80.6  241                                   5-4   14.2     372(2.0)  438      81.9  248                                   5-5   14.5     465(2.5)  486      82.3  248                                   5-6   14.1     558(3.0)  567      82.5  248                                   5-7   14.0     744(4.0)  634      80.2  248                                   ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        OPTIMIZATION WITH                                                             DIFFERENT ALKALIS, WITH AND WITHOUT RE                                                                      Re                                              Experi-      Alkali   Target  Target                                          ment  %      Dopant   ppmw    Level,                                                                              Initial                                                                             Initial                             No.   wAg    Added    Alkali  ppmw  S.sub.40, %                                                                         T.sub.40, °C.                ______________________________________                                        6-1   13.6   None      0      0     74.6  229                                 6-2   14.3   None      0      372*  54.3  236                                 6-3   14.3   Cs       230     0     80.0  242                                 6-4   14.2   Cs       420     372   81.9  248                                 6-5   14.0   Rb       170     0     79.4  238                                 6-6   14.6   Rb       305     372   80.0  246                                 6-7   14.6   K        130     0     79.4  240                                 6-8   14.5   K        200     372   78.1  239                                 6-9   14.4   Na       207     0     76.5  234                                  6-10 14.7   Na        92     372   74.3  246                                  6-11 13.9   Li        40     0     74.8  233                                  6-12 14.2   Li       120     372   63.5  239                                  6-13 13.2   Cs + Rb  160 + 110                                                                             0     79.4  245                                  6-14 13.2   Cs + Rb  160 + 195                                                                             372   81.3  256                                 ______________________________________                                         *2.0 μmoles/g                                                         

I claim:
 1. In a process for the production of ethylene oxide whereinethylene is contacted in the vapor phase with an oxygen-containing gasat ethylene oxide forming conditions at a temperature ranging betweenabout 180° C. and 330° C. in the presence of a silver metal-containingcatalyst, the improvement which comprises using a catalyst comprising acatalytically effective amount of silver, a promoting amount of alkalimetal and a promoting amount of rhenium supported on a suitable support.2. In a process for the production of ethylene oxide wherein ethylene iscontacted in the vapor phase with an oxygen-containing gas at ethyleneoxide forming conditions at a temperature ranging between about 180° C.and 330° C. in the presence of a silver metal-containing catalyst, theimprovement which comprises using a catalyst comprising silver, alkalimetal promoter and from about 0.2 to about 5 millimoles of rheniumpromoter, measured as the metal, per kilogram of total catalyst,supported on a porous, refractory support; the combination of silver,alkali metal promoter, rhenium promoter and support affording a higherselectivity to ethylene oxide at a given oxygen conversion level than isobtained under the same reaction conditions with the same combination ofsilver and support and none or one of the promoters selected fromrhenium and alkali metal.
 3. In a process for the production of ethyleneoxide wherein ethylene is contacted in the vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at atemperature ranging between about 180° C. and 330° C. in the presence ofa fixed bed of silver metal-containing catalyst, the improvement whichcomprises using a catalyst comprising silver, alkali metal promoter andfrom about 0.2 to about 5 millimoles of rhenium, measured as the metal,per kilogram of total catalyst supported on a porous, refractory supportwherein the rhenium is applied to the support in the form of aperrhenate or rhenium heptoxide; the combination of silver, alkali metalpromoter, rhenium and support affording a higher selectivity to ethyleneoxide at a given oxygen conversion level than is obtained under the samereaction conditions with the same combination of silver and support andnone or one of the promoters selected from rhenium and alkali metal. 4.In a process for the production of ethylene oxide wherein ethylene iscontacted in the vapor phase with an oxygen-containing gas at ethyleneoxide forming conditions at a temperature ranging between about 180° C.and 330° C. in the presence of a fixed bed of silver metal-containingcatalyst, the improvement which comprises using a catalyst comprisingsilver, alkali metal promoter and from about 0.2 to about 5 millimolesof rhenium, measured as the metal, per kilogram of total catalyst in aform which is extractable in a dilute (20 millimolar) aqueous solutionof sodium hydroxide, supported on a porous, refractory support; thecombination of silver, alkali metal promoter, rhenium and supportaffording a higher selectively to ethylene oxide at a given oxygenconversion level than is obtained under the same reaction conditionswith the same combination of silver and support and none or one of thepromoters selected from rhenium and alkali metal.
 5. In a process forthe production of ethylene oxide wherein ethylene is contacted in thevapor phase with an oxygen-containing gas at ethylene oxide formingconditions at a temperature ranging between about 180° C. and 330° C. inthe presence of a fixed bed of silver metal-containing catalyst, theimprovement which comprises using a catalyst comprising silver, alkalimetal promoter and from about 0.2 to about 5 millimoles of rhenium,measured as the metal, per kilogram of total catalyst in a form which isextractable in a dilute (20 millimolar) aqueous solution of sodiumhydroxide supported on a porous, refractory support wherein the rheniumis applied to the support in the form of a perrhenate or rheniumheptoxide; the combination of silver, alkali metal promoter, rhenium andsupport affording a higher selectivity to ethylene oxide at a givenoxygen conversion level than is obtained under the same reactionconditions with the same combination of silver and support and none orone of the promoters selected from rhenium and alkali metal.
 6. Theprocess of claims 1, 2, 3, 4 or 5 wherein in the catalyst the supportcomprises alpha alumina.
 7. The process of claim 6 wherein in thecatalyst the alpha alumina has a surface area ranging from about 0.05 toabout 5 m² /g.
 8. The process of claim 7 wherein in the catalyst thesupport surface area ranges from about 0.1 to about 3 m² /g.
 9. Theprocess of claim 8 wherein in the catalyst the silver ranges from about1 to about 25 percent by weight of the total catalyst, and the alkalimetal promoter ranges from about 20 to about 1500 parts by weight,measured as the metal, per million parts by weight of the totalcatalyst.
 10. The process of claim 9 wherein in the catalyst the silverranges from about 5 to about 20 percent by weight of the total catalystand the alkali metal promoter ranges from about 50 to about 1000 partsby weight, measured as the metal, per million parts by weight of thetotal catalyst.
 11. The process of claim 10 wherein in the catalyst thealkali metal and rhenium are found on the surface of the support or onthe surface of the catalyst.
 12. The process of claim 10 wherein in thecatalyst the alkali metal and rhenium are found individually or in amixture thereof on the catalyst, on the support or on both the catalystand the support.
 13. The process of claim 10 wherein the selectivity ismeasured at an oxygen conversion level of about 40% at a gas hourlyspace velocity of about 3300 and when the catalyst has been placed onstream for about 16±4 hours.
 14. The process of claim 1 wherein in thecatalyst said promoting amount of alkali metal and rhenium is such as toprovide a higher selectivity to ethylene oxide at a given oxygenconversion level under a given set of reaction conditions for thecatalyst than for a comparable catalyst without alkali metal and/orrhenium.
 15. The process of claim 14 wherein the selectivity is measuredat an oxygen conversion level of about 40% at a gas hourly spacevelocity of about 3300 and when the catalyst has been placed onstreamfor about 16±4 hours.
 16. The process of claim 1 wherein in the catalystthe silver ranges from about 1 to about 25 percent by weight of thetotal catalyst, the alkali metal promoter ranges from about 20 to about1500 parts by weight, measured as the metal, per million parts by weightof total catalyst and the rhenium ranges from about 0.2 to about 5millimoles of rhenium, measured as the metal, per kilogram of totalcatalyst.
 17. In a process for the production of ethylene oxide whereinethylene is contacted in the vapor phase with an oxygen-containing gasat ethylene oxide forming conditions at a temperature ranging betweenabout 180° C. and 330° C. in the presence of a silver metal-containingcatalyst, the improvement which comprises using a catalyst comprising acatalytically effective amount of silver, a promoting amount of a higheralkali metal comprising potassium, rubidium, cesium, or mixtures thereofand a promoting amount of rhenium supported on a suitable support. 18.In a process for the production of ethylene oxide wherein ethylene iscontacted in the vapor phase with an oxygen-containing gas at ethyleneoxide forming conditions at a temperature ranging between about 180° C.and 330° C. in the presence of a fixed bed of silver metal-containingcatalyst, the improvement which comprises using a catalyst comprising acatalytically effective amount of silver, a promoting amount of a higheralkali metal selected from the group consisting of potassium, rubidium,cesium and mixtures thereof and a promoting amount of rhenium supportedon a porous, refractory support.
 19. In a process for the production ofethylene oxide wherein ethylene is contacted in the vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at atemperature ranging between about 180° C. and 330° C. in the presence ofa fixed bed of silver metal-containing catalyst, the improvement whichcomprises using a catalyst comprising silver, a higher alkali metalpromoter comprising potassium, rubidium, cesium or mixtures thereof andfrom about 0.2 to about 5 millimoles of rhenium promoter measured as themetal, per kilogram of total catalyst supported on a porous, refractorysupport; the combination of silver, higher alkali metal promoter,rhenium promoter and support affording a higher selectivity to ethyleneoxide at a given oxygen conversion level than is obtained under the samereaction conditions with the same combination of silver and support andnone or one of the promoters selected from rhenium and higher alkalimetal.
 20. In a process for the production of ethylene oxide whereinethylene is contacted in the vapor phase with an oxygen-containing gasat ethylene oxide forming conditions at a temperature ranging betweenabout 180° C. and 330° C. in the presence of a fixed bed of silvermetal-containing catalyst, the improvement which comprises using acatalyst comprising silver, a higher alkali metal promoter comprisingpotassium, rubidium, cesium or mixtures thereof and from about 0.2 toabout 5 millimoles of rhenium, measured as the metal, per kilogram oftotal catalyst supported on a porous, refractory support wherein therhenium is applied to the support in the form of a perrhenate or rheniumheptoxide; the combination of silver, higher alkali metal promoter,rhenium and support affording a higher selectivity to ethylene oxide ata given oxygen conversion level than is obtained under the same reactionconditions with the same combination of silver and support and none orone of the promoters selected from rhenium and higher alkali metal. 21.In a process for the production of ethylene oxide wherein ethylene iscontacted in the vapor phase with an oxygen-containing gas at ethyleneoxide forming conditions at a temperature ranging between about 180° C.and 330° C. in the presence of a fixed bed of silver metal-containingcatalyst, the improvement which comprises using a catalyst comprisingsilver, a higher alkali metal promoter comprising potassium, rubidium,cesium or mixtures thereof and from about 0.2 to about 5 millimoles ofrhenium, measured as the metal, per kilogram of total catalyst in a formwhich is extractable in a dilute (20 millimolar) aqueous solution ofsodium hydroxide, supported on a porous, refractory support; thecombination of silver, higher alkali metal promoter, rhenium and supportaffording a higher selectivity to ethylene oxide at a given oxygenconversion level than is obtained under the same reaction conditionswith the same combination of silver and support and none or one of thepromoters selected from rhenium and higher alkali metal.
 22. In aprocess for the production of ethylene oxide wherein ethylene iscontacted in the vapor phase with an oxygen-containing gas at ethyleneoxide forming conditions at a temperature ranging between about 180° C.and 330° C. in the presence of a fixed bed of silver metal-containingcatalyst, the improvement which comprises using a catalyst comprisingsilver, a higher alkali metal promoter comprising potassium, rubidium,cesium or mixtures thereof and from about 0.2 to about 5 millimoles ofrhenium, measured as the metal, per kilogram of total catalyst in a formwhich is extractable in a dilute (20 millimolar) aqueous solution ofsodium hydroxide, supported on a porous, refractory support wherein therhenium is applied to the support in the form of a perrhenate or rheniumheptoxide; the combination of silver, higher alkali metal promoter,rhenium and support affording a higher selectivity to ethylene oxide ata given oxygen conversion level than is obtained under the same reactionconditions with the same combination of silver and support and none orone of the promoters selected from rhenium and higher alkali metal. 23.In a process for the production of ethylene oxide wherein ethylene iscontacted in the vapor phase with an oxygen-containing gas at ethyleneoxide forming conditions at a temperature ranging between about 180° C.and 330° C. in the presence of a fixed bed of silver metal-containingcatalyst, the improvement which comprises using a catalyst comprisingfrom about 1 to about 25 percent by weight of total catalyst of silver,from about 50 to about 1000 parts by weight, measured as the metal, permillion parts by weight of total catalyst of a higher alkali metalpromoter selected from potassium, rubidium, cesium and mixtures thereofand from about 0.2 to about 5 millimoles of rhenium, measured as themetal, per kilogram of total catalyst supported on a porous, refractorysupport comprising alpha alumina having a surface area ranging fromabout 0.1 to about 3 m² /g and a water pore volume ranging from about0.25 to about 0.55 cc/gm; the combination of silver, higher alkalimetal, rhenium and support affording a higher selectivity to ethyleneoxide at a given oxygen conversion level than is obtained under the samereaction conditions with the same combination of silver and support andnone or one of the promoters selected from rhenium and higher alkalimetal.
 24. In a process for the production of ethylene oxide whereinethylene is contacted in the vapor phase with an oxygen-containing gasat ethylene oxide forming conditions at a temperature ranging betweenabout 180° C. and 330° C. in the presence of a fixed bed of silvermetal-containing catalyst, the improvement which comprises using acatalyst comprising from about 1 to about 25 percent by weight of totalcatalyst of silver, from about 50 to about 1000 parts by weight,measured as the metal, per million parts by weight of total catalyst ofa higher alkali metal promoter selected from potassium, rubidium, cesiumand mixtures thereof and from about 0.2 to about 5 millimoles ofrhenium, measured as the metal, per kilogram of total catalyst supportedon a porous, refractory support comprising alpha alumina having asurface area ranging from about 0.1 to about 3 m² /g and a water porevolume ranging from about 0.25 to about 0.55 cc/gm wherein the rheniumis applied to support in the form of a perrhenate or rhenium heptoxide;the combination of silver, higher alkali metal promoter, rhenium andsupport affording a higher selectivity to ethylene oxide at a givenoxygen conversion level than is obtained under the same reactionconditions with the same combination of silver and support and none orone of the promoters selected from rhenium and higher alkali metal. 25.In a process for the production of ethylene oxide wherein ethylene iscontacted in the vapor phase with an oxygen-containing gas at ethyleneoxide forming conditions at a temperature ranging between about 180° C.and 330° C. in the presence of a fixed bed of silver metal-containingcatalyst, the improvement which comprises using a catalyst comprisingfrom about 1 to about 25 percent by weight of total catalyst of silver,from about 50 to about 1000 parts by weight, measured as the metal, permillion parts by weight of total catalyst of a higher alkali metalpromoter selected from potassium, rubidium, cesium and mixtures thereofand from about 0.2 to about 5 millimoles of rhenium, measured as themetal, per kilogram of total catalyst in a form which is extractable ina dilute (20 millimolar) aqueous solution of sodium hydroxide supportedon a porous, refractory support comprising alpha alumina having asurface area ranging from about 0.1 to about 3 m² /g and a water porevolume ranging from about 0.25 to about 0.55 cc/gm; the combination ofsilver, higher alkali metal, rhenium and support affording a higherselectivity to ethylene oxide at a given oxygen conversion level than isobtained under the same reaction conditions with the same combination ofsilver and support and none or one of the promoters selected fromrhenium and higher alkali metal.
 26. In a process for the production ofethylene oxide wherein ethylene is contacted in the vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at atemperature ranging between about 180° C. and 330° C. in the presence ofa fixed bed of silver metal-containing catalyst, the improvement whichcomprises using a catalyst comprising from about 1 to about 25 percentby weight of total catalyst of silver, from about 50 to about 1000 partsby weight, measured as the metal, per million parts by weight of totalcatalyst of a higher alkali metal promoter selected from potassium,rubidium, cesium and mixtures thereof and from about 0.2 to about 5millimoles of rhenium, measured as the metal, per kilogram of totalcatalyst in a form which is extractable in a dilute (20 millimolar)aqueous solution of sodium hydroxide supported on a porous, refractorysupport comprising alpha alumina having a surface area ranging fromabout 0.1 to about 3 m² /g and a water pore volume ranging from about0.25 to about 0.55 cc/g wherein the rhenium is applied to the support inthe form of a perrhenate or rhenium heptoxide; the combination ofsilver, higher alkali metal, rhenium and support affording a higherselectivity to ethylene oxide at a given oxygen conversion level than isobtained under the same reaction conditions with the same combination ofsilver and support and none or one of the promoters selected fromrhenium and higher alkali metal.
 27. The process of claims 17, 18, 19,20, 21 or 22 wherein in the catalyst the support comprises alphaalumina.
 28. The process of claims 17, 18, 19, 20, 21 or 22 wherein inthe catalyst the support comprises alpha alumina which has a surfacearea ranging from about 0.05 to about 5 m² /g and a water pore volumeranging from about 0.10 to about 0.75 cc/g.
 29. The process of claim 28wherein the support surface area ranges from about 0.1 to about 3 m² /g.30. The process of claims 17 or 18 wherein in the catalyst the silverranges from about 1 to about 25 percent by weight of the total catalyst,the higher alkali metal promoter ranges from about 20 to about 1500parts by weight, measured as the metal, per million parts by weight oftotal catalyst, the rhenium ranges from about 0.2 to about 5 millimolesof rhenium, measured as the metal, per kilogram of total catalyst andthe support comprises alpha alumina with a surface area ranging fromabout 0.05 to about 5 m² /g.
 31. The process of claim 30 wherein in thecatalyst said promoting amount of alkali metal and rhenium is such as toprovide a higher selectivity to ethylene oxide at a given oxygenconversion level under a given set of reaction conditions for thecatalyst than for a comparable catalyst without alkali metal and/orrhenium.
 32. The process of claim 31 wherein the selectivity is measuredat an oxygen conversion level of about 40% at a gas hourly specevelocity of about 3300 and when the catalyst has been placed onstreamfor about 16±4 hours.
 33. The catalyst of claim 30 wherein in thecatalyst the higher alkali metal ranges from about 50 to about 1000 ppmby weight.
 34. The process of claim 33 wherein the higher alkali metalcomprises potassium.
 35. The process of claim 33 wherein in the catalystthe higher alkali metal comprises rubidium.
 36. The process of claim 33wherein in the catalyst the higher alkali metal comprises cesium. 37.The process of claim 36 wherein in the catalyst the higher alkali metalcomprises potassium and cesium.
 38. The process of claim 36 wherein inthe catalyst the higher alkali metal comprises rubidium and cesium. 39.The process of claim 36 wherein in the catalyst the higher alkali metalcomprises cesium, rubidium and potassium.
 40. The process of claim 36wherein in the catalyst the alkali metal and rhenium are found on thesurface of the support or on the surface of the catalyst.
 41. Theprocess of claim 36 wherein in the catalyst the alkali metal and rheniumare found individually or in a mixture thereof on the catalyst, on thesupport or on both the catalyst and the support.
 42. The process ofclaims 19, 20, 21 or 22 wherein in the catalyst the silver ranges fromabout 1 to about 25 percent by weight of the total catalyst and thealkali metal promoter ranges from about 20 to about 1500 parts byweight, measured as the metal, per million parts by weight of the totalcatalyst.
 43. The process of claim 42 wherein in the catalyst the silverranges from about 5 to about 20 percent by weight of the total catalystand the alkali metal promoter ranges from about 50 to about 1000 partsby weight, measured as the metal, per million parts by weight of thetotal catalyst.
 44. The process of claim 43 wherein in the catalyst thealkali metal comprises potassium.
 45. The process of claim 43 wherein inthe catalyst the alkali metal comprises rubidium.
 46. The process ofclaim 43 wherein in the catalyst the alkali metal comprises cesium. 47.The process of claim 43 wherein in the catalyst the alkali metal andrhenium are found on the surface of the support or on the surface of thecatalyst.
 48. The process of claim 43 wherein in the catalyst the alkalimetal and rhenium are found individually or in a mixture thereof on thecatalyst, on the support or on both the catalyst and the support. 49.The process of claim 43 wherein in the catalyst the selectivity ismeasured at an oxygen conversion level of about 40% at a gas hourlyspace velocity of about 3300 and when the catalyst has been placedonstream for about 16±4 hours.
 50. The process of claims 23, 24, 25 or26 wherein in the catalyst the silver ranges from about 5 to about 20percent by weight.
 51. The process of claims 23, 24, 25 or 26 wherein inthe catalyst the alkali metal and rhenium are found on the surface ofthe support.
 52. The process of claims 23, 24, 25 or 26 wherein in thecatalyst the alkali metal and rhenium are found on the surface of thecatalyst.
 53. The process of claims 23, 24, 25 or 26 wherein in thecatalyst the alkali metal and rhenium are found individually or in amixture thereof on the catalyst, on the support or on both the catalystand the support.
 54. The process of claim 53 wherein the selectivity ismeasured at an oxygen conversion level of about 40% at a gas hourlyspace velocity of about 3300 and when the catalyst has been placedonstream for about 16±4 hours.
 55. The process of claim 50 wherein inthe catalyst the higher alkali metal comprises potassium.
 56. Theprocess of claim 50 wherein in the catalyst the higher alkali metalcomprises rubidium.
 57. The process of claim 50 wherein in the catalystthe higher alkali metal comprises cesium.
 58. In a process for theproduction of ethylene oxide wherein ethylene is contacted in the vaporphase with an oxygen-containing gas at ethylene oxide forming conditionsat a temperature ranging between about 180° C. and 330° C. in thepresence of a silver metal-containing catalyst, the improvement whichcomprises using a catalyst comprising a catalytically effective amountof silver, a promoting amount of cesium and a promoting amount ofrhenium on a porous refractory support comprising alpha alumina.
 59. Ina process for the production of ethylene oxide wherein ethylene iscontacted in the vapor phase with an oxygen-containing gas at ethyleneoxide forming conditions at a temperature ranging between about 180° C.and 330° C. in the presence of a fixed bed of silver metal-containingcatalyst, the improvement which comprises using a catalyst comprisingfrom about 1 to about 25 percent by weight of total catalyst of silver,from about 50 to about 1000 parts by weight, measured as the metal, permillion parts by weight of total catalyst of cesium promoter and fromabout 0.2 to about 5 millimoles of rhenium, measured as the metal, perkilogram of total catalyst supported on a porous, refractory supportcomprising alpha alumina having a surface area ranging from about 0.1 toabout 3 m² /g and a water pore volume ranging from about 0.25 to about0.55 cc/gm; the combination of silver, cesium promoter, rhenium andsupport affording a higher selectivity to ethylene oxide at a givenoxygen conversion level than is obtained under the same reactionconditions with the same combination of silver and support and none orone of the promoters selected from rhenium and cesium.
 60. In a processfor the production of ethylene oxide wherein ethylene is contacted inthe vapor phase with an oxygen-containing gas at ethylene oxide formingconditions at a temperature ranging between about 180° C. and 330° C. inthe presence of a fixed bed of silver metal-containing catalyst, theimprovement which comprises using a catalyst comprising from about 1 toabout 25 percent by weight of total catalyst of silver, from about 50 toabout 1000 parts by weight, measured as the metal, per million parts byweight of total catalyst of cesium promoter and from about 0.2 to about5 millimoles of rhenium, measured as the metal, per kilogram of totalcatalyst supported on a porous refractory support comprising alphaalumina having a surface area ranging from about 0.1 to about 3 m² /gand a water pore volume ranging from about 0.25 to about 0.55 cc/gwherein the rhenium is applied to the support as rhenium heptoxide orammonium or alkali metal perrhenate or mixtures thereof; the combinationof silver, cesium promoter, rhenium and support affording a higherselectivity to ethylene oxide at a given oxygen conversion level than isobtained under the same reaction conditions with the same combination ofsilver and support and none or one of the promoters selected fromrhenium and cesium.
 61. In a process for the production of ethyleneoxide wherein ethylene is contacted in the vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at atemperature ranging between about 180° C. and 330° C. in the presence ofa fixed bed of silver metal-containing catalyst, the improvement whichcomprises using a catalyst comprising from about 1 to about 25 percentby weight of total catalyst of silver, from about 50 to about 1000 partsby weight, measured as the metal, per million parts by weight of totalcatalyst of cesium promoter and from about 0.2 to about 5 millimoles ofrhenium, measured as the metal, per kilogram of total catalyst in a formwhich is extractable in a dilute (20 millimolar) aqueous solution ofsodium hydroxide supported on a porous, refractory support comprisingalpha alumina having a surface area ranging from about 0.1 to about 3 m²/g and a water pore volume ranging from about 0.25 to about 0.55 cc/gm;the combination of silver, cesium promoter, rhenium and supportaffording a higher selectivity to ethylene oxide at a given oxygenconversion level than is obtained under the same reaction conditionswith the same combination of silver and support and none or one of thepromoters selected from rhenium and cesium.
 62. In a process for theproduction of ethylene oxide wherein ethylene is contacted in the vaporphase with an oxygen-containing gas at ethylene oxide forming conditionsat a temperature ranging between about 180° C. and 330° C. in thepresence of a fixed bed of silver metal-containing catalyst, theimprovement which comprises using a catalyst comprising from about 1 toabout 25 percent by weight of total catalyst of silver, from about 50 toabout 1000 parts by weight, measured as the metal, per million parts byweight of total catalyst of cesium promoter and from about 0.2 to about5 millimoles of rhenium, measured as the metal, per kilogram of totalcatalyst in a form which is extractable in a dilute (20 millimolar)aqueous solution of sodium hydroxide supported on a porous refractorysupport comprising alpha alumina having a surface area ranging fromabout 0.1 to about 3 m² /g and a water pore volume ranging from about0.25 to about 0.55 cc/g wherein the rhenium is applied to the support asrhenium heptoxide or ammonium or alkali metal perrhenate or mixturesthereof; the combination of silver, cesium promoter, rhenium and supportaffording a higher initial selectivity to ethylene oxide at a givenoxygen conversion level than is obtained under the same reactionconditions with the same combination of silver and support and none orone of the promoters selected from rhenium and cesium.
 63. The processof claim 58 wherein in the catalyst the silver ranges from about 1 toabout 25 percent by weight of the total catalyst, the cesium ranges fromabout 20 to about 1500 parts by weight, measured as the metal, permillion parts by weight of the total catalyst and the rhenium rangesfrom about 0.2 to about 5 millimoles of rhenium, measured as the metal,per kilogram of total catalyst.
 64. The process of claim 63 wherein inthe catalyst the cesium ranges from about 50 to about 1000 ppm.
 65. Theprocess of claim 64 wherein in the catalyst the support has a surfacearea ranging from about 0.05 to about 5 m² /g.
 66. The process of claim65 wherein in the catalyst the support surface area ranges from about0.1 to about 3 m² /g.
 67. The process of claim 58 wherein in thecatalyst said promoting amount of cesium and rhenium is such as toprovide a higher selectivity to ethylene oxide at a given oxygenconversion level under a given set of reaction conditions for thecatalyst than for a comparable catalyst without cesium and/or rhenium.68. The process of claims 59, 60, 61, 62 or 67 wherein the selectivityis measured at an oxygen conversion level of about 40% at a gas hourlyspace velocity of about 3300 and when the catalyst has been placedonstream for about 16±4 hours.
 69. The process of claims 58, 59, 60, 61or 63 wherein in the catalyst the cesium and rhenium are found on thesurface of the catalyst.
 70. The process of claims 58, 59, 60, 61 or 62wherein in the catalyst the cesium and rhenium are found on the surfaceof the support.
 71. The process of claims 58, 59, 60, 61 or 62 whereinin the catalyst the cesium and rhenium are found individually or in amixture thereof on the catalyst, on the support or on both the catalystand the support.
 72. The process of claim 71 wherein the selectivity ismeasured at an oxygen conversion level of about 40% at a gas hourlyspace velocity of about 3300 and when the catalyst has been placedonstream for about 16±4 hours.
 73. In a process for the production ofethylene oxide wherein ethylene is contacted in the vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at atemperature ranging between about 180° C. and 330° C. in the presence ofa silver metal-containing catalyst, the improvement which comprisesusing a catalyst which comprises impregnating a porous refractorysupport with a catalytically effective amount of silver, a promotingamount of alkali metal and a promoting amount of rhenium.
 74. In aprocess for the production of ethylene oxide wherein ethylene iscontacted in the vapor phase with an oxygen-containing gas at ethyleneoxide forming conditions at a temperature ranging between about 180° C.and 330° C. in the presence of a fixed bed of silver metal-containingcatalyst, the improvement which comprises using a catalyst prepared by aprocess of which comprises impregnating a porous refractory support withone or more solutions comprising silver, alkali metal and/or rheniumwherein the concentration of the silver (measured as the metal) in thesolution ranges from about 1 g/l to the solubility limit of silver inthe solution, the concentration of alkali metal (measured as the metal)in the solution ranges from about 1×10⁻³ g/l to about 12 g/l and theconcentration of the rhenium (measured as the metal) ranges from about 5×10⁻³ g/l to about 20 g/l to provide the catalyst with a catalyticallyeffective amount of silver, a promoting amount of alkali metal and apromoting amount of rhenium.
 75. In a process for the production ofethylene oxide wherein ethylene is contacted in the vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at atemperature ranging between about 180° C. and 330° C. in the presence ofa fixed bed of silver metal-containing catalyst, the improvement whichcomprises using a catalyst which comprises using a catalyst prepared bya process which comprises impregnating a porous refractory support withone or more solutions comprising solvent having silver compound(s)dissolved therein, and/or alkali metal compound(s) dissolved thereinand/or rhenium-containing compound(s) dissolved therein sufficient todeposit on the support from about 1 to about 25 percent by weight oftotal catalyst of silver compound(s), measured as the metal, from about20 to about 1500 ppm by weight of alkali metal compound(s), measured asthe metal and from about 0.2 to about 5 millimoles per kilogram of totalcatalyst, measured as the metal, of rhenium-containing compound(s) toprovide the catalyst with a catalytically effective amount of silver, apromoting amount of alkali metal and a promoting amount of rhenium. 76.In a process for the production of ethylene oxide wherein ethylene iscontacted in the vapor phase with an oxygen-containing gas at ethyleneoxide forming conditions at a temperature ranging between about 180° C.and 330° C. in the presence of a fixed bed of silver metal-containingcatalyst, the improvement which comprises using a catalyst prepared by aprocess which comprises impregnating a porous, refractory support withone or more solutions comprising silver ions, alkali metal ions,rhenium-containing ions or mixtures thereof sufficient to deposit on thecarrier from about 1 to about 25 percent by weight of total catalyst ofsilver, from about 20 to about 1500 parts by weight, measured as themetal, per million parts by weight of total catalyst, of alkali metalcompound(s), and from about 0.2 to about 5 millimoles ofrhenium-containing compound(s), measured as the metal, per kilogram oftotal catalyst, and after impregnation, reducing the silver on thesupport to metallic silver to cause the combination of silver, alkalimetal promoter, rhenium and support to have a higher selectivity toethylene oxide at a given oxygen conversion level than is obtained underthe same reaction conditions with the same combination of silver andsupport and none or one of the promoters selected from rhenium andalkali metal.
 77. In a process for the production of ethylene oxidewherein ethylene is contacted in the vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at atemperature ranging between about 180° C. and 330° C. in the presence ofa fixed bed of silver metal-containing catalyst, the improvement whichcomprises using a catalyst prepared by a process which comprisesimpregnating a porous, refractory support with one or more solutionscomprising silver ions, alkali metal ions, rhenium-containing ions ormixtures thereof sufficient to deposit on the support from about 1 toabout 25 percent by weight of total catalyst of silver, from about 20 toabout 1500 parts by weight, measured as the metal, per million parts byweight of total catalyst, of alkali metal compound(s), and from about0.2 to about 5 millimoles of rhenium compound(s), measured as the metal,per kilogram of total catalyst, said rhenium compound(s) providing inthe final catalyst rhenium in a form which is extractable in a dilute(20 millimolar) aqueous sodium hydroxide solution; and afterimpregnation, reducing the silver on the support to metallic silver tocause the combination of silver, alkali metal promoter, rhenium andsupport to have a higher selectivity to ethylene oxide at a given oxygenconversion level than is obtained under the same reaction conditionswith the same combination of silver and support and none or one of thepromoters selected from rhenium and alkali metal.
 78. In a process forthe production of ethylene oxide wherein ethylene is contacted in thevapor phase with an oxygen-containing gas at ethylene oxide formingconditions at a temperature ranging between about 180° C. and 330° C. inthe presence of a silver metal-containing catalyst, the improvementwhich comprises using a catalyst prepared by a process production ofethylene oxide from ethylene and molecular oxygen which processcomprises impregnating a porous refractory support comprising alphaalumina with a catalytically effective amount of silver, a promotingamount of higher alkali metal comprising potassium, rubidium, cesium ormixtures thereof and a promoting amount of rhenium.
 79. In a process forthe production of ethylene oxide wherein ethylene is contacted in thevapor phase with an oxygen-containing gas at ethylene oxide formingconditions at a temperature ranging between about 180° C. and 330° C. inthe presence of a fixed bed of silver metal-containing catalyst, theimprovement which comprises using a catalyst prepared by a process whichcomprises impregnating a porous refractory support comprising alphaalumina with one or more solutions comprising silver, and/or higheralkali metal selected from the group consisting of potassium, rubidium,cesium and mixtures thereof, and/or rhenium wherein the concentration ofthe silver (measured as the metal) in the solution ranges from about 1g/l to the solubility limit of silver in the solution, the concentrationof alkali metal (measured as the metal) in the solution ranges fromabout 1×10⁻³ g/l to about 12 g/l and the concentration of the rhenium(measured as the metal) ranges from about 5×10⁻³ g/l to about 20 g/l toprovide the catalyst with a catalytically effective amount of silver, apromoting amount of higher alkali metal and a promoting amount ofrhenium.
 80. In a process for the production of ethylene oxide whereinethylene is contacted in the vapor phase with an oxygen-containing gasat ethylene oxide forming conditions at a temperature ranging betweenabout 180° C. and 330° C. in the presence of a fixed bed of silvermetal-containing catalyst, the improvement which comprises using acatalyst prepared by a process which comprises impregnating a porousrefractory support comprising alpha alumina with one or more solutionscomprising solvent having silver compound(s) dissolved therein, and/orhigher alkali metal compound(s) selected from compound(s) of potassium,rubidium, cesium and mixtures thereof dissolved therein and/orrhenium-containing compound(s) dissolved therein sufficient to depositon the support from about 1 to about 25 percent by weight of totalcatalyst of silver compound(s), measured as the metal, from about 20 toabout 1500 ppm by weight of higher alkali metal compound(s), measured asthe metal, and from about 0.2 to about 5 millimoles per kilogram oftotal catalyst of rhenium-containing compound(s), measured as the metal,to provide a catalyst with a catalytically effective amount of silver, apromoting amount of higher alkali metal and a promoting amount ofrhenium.
 81. In a process for the production of ethylene oxide whereinethylene is contacted in the vapor phase with an oxygen-containing gasat ethylene oxide forming conditions at a temperature ranging betweenabout 180° C. and 330° C. in the presence of a fixed bed of silvermetal-containing catalyst, the improvement which comprises using acatalyst prepared by a a process which comprises impregnating a porous,refractory support comprising alpha alumina with one or more solutionscomprising silver ions, higher alkali metal ions selected from the groupconsisting of ions of potassium, rubidium, cesium and mixtures thereof,rhenium-containing ions or mixtures thereof sufficient to deposit on thesupport from about 1 to about 25 percent by weight of total catalyst ofsilver, from about 20 to about 1500 parts by weight, measured as themetal, per million parts by weight of total catalyst, of higher alkalimetal compound(s) selected from compound(s) of potassium, rubidium,cesium and mixtures thereof, and from about 0.2 to about 5 millimoles ofrhenium-containing compound(s), measured as the metal, per kilogram oftotal catalyst, and after impregnation, reducing the silver on thesupport to metallic silver to cause the combination of silver, higheralkali metal promoter, rhenium and support to have a higher selectivityto ethylene oxide at a given oxygen conversion level than is obtainedunder the same reaction conditions with the same combination of silverand support and none or one of the promoters selected from rhenium andhigher alkali metal.
 82. In a process for the production of ethyleneoxide wherein ethylene is contacted in the vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at atemperature ranging between about 180° C. and 330° C. in the presence ofa fixed bed of silver metal-containing catalyst, the improvement whichcomprises using a catalyst prepared by a process which comprisesimpregnating a porous, refractory support comprising alpha alumina withone or more solutions comprising silver ions, higher alkali metal ionsselected from the group consisting of ions of potassium, rubidium,cesium and mixtures thereof, rhenium-containing ions or mixtures thereofsufficient to deposit on the support from about 1 to about 25 percent byweight of total catalyst of silver, from about 20 to about 1500 parts byweight, measured as the metal, per million parts by weight of totalcatalyst, of higher alkali metal compound(s) selected from compound(s)of potassium, rubidium, cesium and mixtures thereof, and from about 0.2to about 5 millimoles of a rhenium-containing compound(s), measured asthe metal, per kilogram of total catalyst, said rhenium-containingcompound(s) providing in the final catalyst rhenium in a form which isextractable in a dilute (20 millimolar) aqueous sodium hydroxidesolution; and after impregnation, reducing the silver on the support tometallic silver to cause the combination of silver, higher alkali metal,rhenium and support to have a higher selectivity to ethylene oxide at agiven oxygen conversion level than is obtained under the same reactionconditions with the same combination of silver and support and none orone of the promoters selected from rhenium and higher alkali metal. 83.In a process for the production of ethylene oxide wherein ethylene iscontacted in the vapor phase with an oxygen-containing gas at ethyleneoxide forming conditions at a temperature ranging between about 180° C.and 330° C. in the presence of a silver metal-containing catalyst, theimprovement which comprises using a catalyst prepared by a process whichcomprises impregnating a porous refractory support comprising alphaalumina with a catalytically effective amount of silver, a promotingamount of cesium and a promoting amount of rhenium.
 84. In a process forthe production of ethylene oxide wherein ethylene is contacted in thevapor phase with an oxygen-containing gas at ethylene oxide formingconditions at a temperature ranging between about 180° C. and 330° C. inthe presence of a fixed bed of silver metal-containing catalyst, theimprovement which comprises using a catalyst prepared by a processcomprises impregnating a porous refractory support comprising alphaalumina with one or more solutions comprising silver, and/or cesiumand/or rhenium wherein the concentration of the silver (measured as themetal) in the solution ranges from about 1 g/l to the solubility limitof silver in the solution, the concentration of cesium (measured as themetal) in the solution ranges from about 1×10⁻³ g/l to about 12 g/l andthe concentration of the rhenium (measured as the metal) ranges fromabout 5×10⁻³ g/l to about 20 g/l to provide the catalyst with acatalytically effective amount of silver, a promoting amount of cesiumand a promoting amount of rhenium.
 85. In a process for the productionof ethylene oxide wherein ethylene is contacted in the vapor phase withan oxygen-containing gas at ethylene oxide forming conditions at atemperature ranging between about 180° C. and 330° C. in the presence ofa fixed bed of silver metal-containing catalyst, the improvement whichcomprises using a catalyst prepared by a process which comprisesimpregnating a porous refractory support comprising alpha alumina withone or more solutions comprising solvent having silver compound(s)dissolved therein, and/or cesium compound(s) dissolved therein and/orrhenium-containing compound(s) dissolved therein sufficient to depositon the support from about 1 to about 25 percent by weight of totalcatalyst of silver compound(s), measured as the metal, from about 20 toabout 1500 ppm by weight of cesium compound(s) measured as the metal,and from about 0.2 to about 5 millimoles per kilogram of total catalystof rhenium-containing compound(s), measured as the metal, to provide thecatalyst with a catalytically effective amount of silver, a promotingamount of cesium and a promoting amount of rhenium.
 86. In a process forthe production of ethylene oxide wherein ethylene is contacted in thevapor phase with an oxygen-containing gas at ethylene oxide formingconditions at a temperature ranging between about 180° C. and 330° C. inthe presence of a fixed bed of silver metal-containing catalyst, theimprovement which comprises using a catalyst prepared by a process whichcomprises impregnating a porous, refractory support comprising alphaalumina with one or more solutions comprising solvent silver ions,cesium ions, rhenium-contianing ions or mixtures thereof sufficient todeposit on the support from about 1 to about 25 percent by weight oftotal catalyst of silver, from about 20 to about 1500 parts by weight,measured as the metal, per million parts by weight of total catalyst, ofcesium and from about 0.2 to about 5 millimoles of rhenium-containingcompound(s), measured as the metal, per kilogram of total catalyst, andafter impregnation, reducing the silver on the support to metallicsilver to cause the combination of silver, cesium, rhenium and supportto have a higher selectivity to ethylene oxide at a given oxygenconversion level than is obtained under the same reaction conditionswith the same combination of silver and support and none or one of thepromoters selected from rhenium and cesium.
 87. In a process for theproduction of ethylene oxide wherein ethylene is contacted in the vaporphase with an oxygen-containing gas at ethylene oxide forming conditionsat a temperature ranging between about 180° C. and 330° C. in thepresence of a fixed of silver metal-containing catalyst, the improvementwhich comprises using a catalyst prepared by a process which comprisesimpregnating a porous refractory support comprising alpha alumina withone or more solutions comprising silver ions, cesium ions,rhenium-containing ions or mixtures thereof sufficient to deposit on thesupport from about 1 to about 25 percent by weight of total catalyst ofsilver, from about 20 to about 1500 parts by weight, measured as themetal, per million parts by weight of total catalyst, of cesiumcompound(s), and from about 0.2 to about 5 millimoles ofrhenium-containing compound(s), measured as the metal, per kilogram oftotal catalyst, said rhenium-containing compound(s) providing in thefinal catalyst rhenium in a form which is extractable in a dilute (20millimolar) aqueous sodium hydroxide solution; and after impregnation,reducing the silver on the support to metallic silver to cause thecombination of silver, cesium, rhenium and support to have a higherselectivity to ethylene oxide at a given oxygen conversion level than isobtained under the same reaction conditions with the same combination ofsilver and support and none or one of the promoters selected fromrhenium and cesium.
 88. The process of claims 83, 74, 78, 79, 88 or 84wherein in the process for preparing the catalyst the amount of silveradded by impregnation ranges from about 1 to about 25 percent by weight,the amount of alkali metal added by impregnation ranges from about 20 toabout 1500 ppm by weight, measured as the metal, and the amount ofrhenium added by impregnation ranges from about 0.2 to about 5millimoles of rhenium, measured as the metal, per kilogram of totalcatalyst.
 89. The process of claims 73, 74, 78, 79, 83, or 84 wherein inthe process for preparing the catalyst the amount of silver found on thecatalyst ranges from about 1 to about 25 percent by weight, the amountof alkali metal found on the catalyst ranges from about 20 to about 1500ppm by weight, measured as the metal, and the amount of rhenium found onthe catalyst ranges from about 0.2 to about 5 millimoles of rheniummeasured as the metal, per kilogram of total catalyst.
 90. The processof claims 73, 74, 78, 79, 83 or 84 wherein in the process for preparingthe catalyst the amount of silver found on the surface of the supportranges from about 1 to about 25 percent by weight, the amount of alkalimetal found on the surface of the support ranges from about 20 to about1500 ppm by weight, measured as the metal, and the amount of rheniumfound on the surface of the support ranges from about 0.2 to about 5millimoles of rhenium, measured as the metal, per kilogram of totalcatalyst.
 91. The process of claims 73, 74, 75, 78, 79, 80, 83, 84 or 85wherein in the process for preparing the catalyst after impregnation thesilver is reduced to metallic silver.
 92. The process as in any ofclaims 73-87 wherein in the process for preparing the catalyst afterimpregnation the silver is reduced to metallic silver by heating at atemperature between about 75° C. to about 400° C.
 93. The process as inany of claims 73-87 wherein in the process for preparing the catalystafter impregnation the silver is reduced to metallic silver by heatingat a temperature between about 50° C. to about 600° C.
 94. The processof claims 74, 75, 76, 77, 79, 80, 81, 82, 84, 85, 86 or 87 wherein inthe process for preparing the catalyst the solution containing silveralso comprises water and vicinal alkylenediamine(s) of from 2 to 4carbon atoms.
 95. The process of claims 74, 75, 76, 77, 79, 80, 81, 82,84, 85, 86 or 87 wherein in the process for preparing the catalyst thesolution containing silver also comprises water and ethylenediamine. 96.The process of claims 74, 75, 76, 77, 79, 80, 81, 82, 84, 85, 86 or 87wherein in the process for preparing the catalyst the solutioncontaining silver also comprises water, vicinal alkalinediamine(s) offrom 2 to 4 carbon atoms and vicinal alkanolamine(s) of from 2 to 4carbon atoms.
 97. The process of claims 74, 75, 76, 77, 79, 80, 81, 82,84, 85, 86 or 87 wherein the solution containing silver also compriseswater, ethylenediamine and monoethanolamine.
 98. The process of claims75, 80 or 85 wherein in the process for preparing the catalyst thesilver compound is selected from silver oxalate, silver oxide, silvercarbonate, silver lactate and mixtures thereof.
 99. The process ofclaims 75, 80 or 85 wherein in the process for preparing the catalystthe alkali metal compound is a hydroxide and/or a nitrate.
 100. Theprocess of claims 75, 80 or 85 wherein in the process for preparing thecatalyst the rhenium-containing compound is rhenium heptoxide orammonium and/or alkali metal perrhenate.
 101. The process of claims 76,77, 81, 82, 86 or 87 wherein in the process for preparing the catalystthe rhenium-containing ions are perrhenate ions.
 102. The process ofclaims 76, 77, 81, 82, 86 or 87 wherein in the process for preparing thecatalyst the selectivity is measured at an oxygen conversion of about40% at a gas hourly space velocity of about 3300 and when the catalysthas been placed onstream for about 16±4 hours.
 103. The process ofclaims 1, 17, 18, 58, 73, 78 or 83 wherein in the catalyst or in theprocess for preparing the catalyst the support has a surface arearanging from about 0.05 to about 10 m² /g.